Treating trans men as near identical to cis men in every context isn't being a good ally to us, actually.
Like yes, treat us with respect, recognize us as men and be normal about it (without stereotyping, alienating, or ostracizing us), but if treating us "as men" means not recognizing us as *trans* men even in contexts where that really matters, if you can only respect our identities when treating us uniformly to how you would *cis* men, then you're not a good trans ally.
Being a good trans ally means actively listening to us to understand how our transness influences our personal experiences, our sociopolitical positions, our interactions with gendered concepts, with manhood itself and what society expects and rejects in us as men- and being individuals who moved away from/rejected the womanhood and traditional femininity expected of us.
To walk into every conversation about trans men with the attitude "well you're a man so I'll treat you just as I would any man (aka cis men)" is obtusely overly simplistic at best, and outright degendering and dehumanizing at worst- denying us recognition without the accusation or implication we are misgendering ourselves when our experiences differ from cis men.
We are men and we aren't cis men, and acknowledging that is not misgendering, it is not insidious to bring up, it is not obfuscating to talking about us as men, it is just actual, literal reality.
Sanding away fundamental aspects of our experience to cram us into the boxes you expect of cis men is not being a good trans ally. It feels obvious but for some reason it just isn't for some people. You can absolutely respect us as men while understanding we are transgender men, and not just 'transgender' as a separate marginalizing qualifier tacked on to someone who would be otherwise identical to a cis man if you took that away.
That's not how this works. These two concepts literally cannot exist without the other, or we would not be who we are. Our transness is not separate or incidental to our manhood and our experiences related to manhood. Our manhood is not incidental to our transness and our experiences related to being trans. You need to respect us in our culmination, as a unique and valid expression of manhood, as a unique and important expression of transgender existence.
Now of course, an individual trans man may not want to be referred to as a trans man, just as a man, and you should respect that. In day to day, a trans man might not want you to treat him any differently than you would a cis man, and you should respect that. That's not the sort of thing I'm talking about here.
It's that when it comes to recognizing how we must navigate society, patriarchy, gender dynamics, and gendered oppression- erasing the reality of our transness to treat us as functionally cis men besides a few quirks is not at all showing solidarity to us, but ignorance. It is a flattening of who and what we are and what we go through.
This kind of attitude is not helpful or supportive, it impedes our conversations about our lives, our rights, and our needs which are unique to us- unique needs which are non-delegitimizing to our identities as men, no matter if they divert from what cis men experience. These needs not only deserve recognition- but any proper allyship requires engaging with these important aspects of our reality.