'It' isn't focused exclusively at binary trans people- it's aimed at androgyny/"gender ambiguity" (real or perceived) of any sort. The issue in regards to binary genders is that society doesn't accept them as men and women. 'It' when used at transgendered people (in general or a specific one) is saying "Well, you're no longer a [man/woman] and I refuse to accept you as a [woman/man]". It's just another form of misgendering.
This is not the same as how trans men can't reclaim "tranny", which gudbuytjane described as:
When are trans men going to realize the only claim they have against ‘tranny’ is the first four letters? And when will cis* people realize the claim they have against it is nothing? When you spell it out it’s a slur against trans women, and this insistence on forcing it into use shows the depth of transmisogyny in the queer community. You are not ‘reclaiming’ it for yourself, you are enforcing it for cis* dominance, and at the expense of trans women
'It' isn't often directed at what society accepts as men or women (the irony here being that, in the same breath, they don't accept than anyone isn't a man or a woman)- it's pretty much a slur against people who aren't, and people who society doesn't want to accept as binary (because they don't fit into the cis-binary) get caught in the crossfires. Non-binaries aren't reclaiming a word that has nothing to do with us- we're reclaiming a word that has everything to do with us.
The entire premise of "it" as an insult is basically 1. "Everyone is a boy or a girl- so this person must be one or the other and calling them anything else is an insult" (and because you can't tell which one they are, only 'it' is an acceptable insult- we can't correctly gender people by accident!) 2. "There's something wrong with anyone who doesn't make it obvious whether they're a boy or a girl (or is "really" a boy/girl but are trying to make us think they're a girl/boy)" 3. "Because of whatever's wrong with them, they don't deserve my respect and I'm in the right to treat them as sub-human". What does this say about people who aren't either a man or a woman? It says that 1. that gender is wrong/non-existant/unacceptable 2. There is something wrong with non-binaries if they act on that gender 3. they don't deserve respect for their gender(s). What does it say about binary genders? "We don't want to accept your gender- we consider you to be your birth sex and pretending to be something else".
Using 'it' as an insult is similar to using tranny and she-male as insults against cis-people. It is using the identities of people who aren't either male or female as an insult and, in doing so, implies that being non-binary (or like a non-binary) in itself is an insult.
Yes, 'it' is thrown against binary trans people. It's thrown against anyone who doesn't fit into the rigid cisbinary view of gender roles. "It" against binary-gendered people is an act of willful misgendering. It shouldn't be anything more or less than calling a trans woman who you know is a woman "man" or "he" or "sir" (a title of respect towards men), because it's the same thing as that. The current system of allowing 'it' to be more offensive than other misgendering is saying that it's worse to be associated with anyone who isn't either male or female and agreeing with the binarist view that people who aren't either male or female are inhuman.
Now you can argue "oh- but 'it' is an inanimate pronoun!", this is a chunk of the reason why not all non-binaries are comfortable using it, but grammar itself is so damned strange. Especially english grammar. People can apply gender to many things- it's more common to call a boat 'she' than 'it'. And hten you get into animate non-human/non-adult nouns which sort of act as the grey area. It's acceptable to call a female cat either 'it' or 'she'- and calling animals 'it' doesn't mean you consider animals to not deserve rights. It's also acceptable to call a pre-pubescent child 'it'- most commonly with "Is it a boy or a girl?", but I've seen it even applied to older children of a known gender. This isn't done with disrespect, often it's used even by people who have a great love for children, we may consider children to be "less-than" but we don't usually consider them to be inhuman. So why is it not allowed to be used for post-pubescent humans? Because we consider them to now be "fully" male or female- because we deny the possibility of them to be anything else.
Rather than getting upset at being called 'it' because 'it' is so inherently insulting- try getting upset for the same reason you get upset when anyone misgenders you in any way. Get upset because misgendering ignores who you know yourself to be and denies your right to be called and treated how you want. All misgendering of transgendered people has the same implications. Whether it's misgendering through 'he' or 'she' or 'it'. The only reason why we get put-off by 'it' is because about 99.5% of the time, 'it' is misgendering so it's used against everyone. As I said- we can't accidentally be correctly gendering people!
No, not all non-binaries are comfortable being called 'it'. Just like not all humans are comfortable being called 'he'. This is why it's vital to ask each individual which pronouns that person would prefer rather than assuming. But just like there are trans women who are reclaiming the word 'tranny' by using it for themselves, there are non-binaries who are reclaiming 'it' and any other word that implies our gender is less acceptable. And we have the right to do that.
The current system of insisting that 'it' is insulting because it inherently denies your humanity (not because the people who use it as a misgendering-insult are denying your humanity- because the pronoun itself does) is extremely insulting to anyone who prefers the pronoun 'it' be used for itself. We shouldn't be saying that a person's preferred pronoun denies its own humanity. Because it doesn't. Using the wrong pronoun does that, no matter what pronoun is right.