When a cis man gets offended and says, "Not all men," it is generally in the context that he feels he is personally being attacked as an individual, and said to try and halt a conversation on sociopolitical issues that he is not generally a victim of (benefits from, even).
When transmascs get offended and say, "Not all men," it is generally in the context of them feeling erased from a sociopolitical perspective on the basis of their nuanced identity (not a cis man but not a woman-- or, in the case of some genderfluid / multigender individuals, not just a woman), and said to try and broaden a conversation on sociopolitical issues (because they are far deeper than just, "masculinity as a whole is evil, and men are trash") that they are being othered from, despite facing said sociopolitical issues as a victim of them.
Whether you acknowledge this or not, having the same exact negative reaction to both instances is a reflection of a cisgender-centric worldview.
