I mean, a passing trans man probably does have it to some extent, but that doesn't necessarily override the substantial disadvantage being trans places.

I mean...when these people talk about discrimination against out trans men, they are talking only about misogyny, not transphobia, and they do not acknowledge that there are people who specifically hate trans men and no other kinds of trans people.

Furthermore, there are advantages and disadvantages to being both a man and a woman. A woman is considered weak and helpless, which means she will be condescended to and discouraged from certain professions, but it also means she is likely to be rescued by do-gooders (sometimes against her will) when she is perceived to be in trouble. Men, by contrast, are considered threatening, and their problems are sometimes ignored, as they are considered strong and capable. To say that men are universally privileged, and that such privileges to not have a cost, is sort of ignorant.

If you're a passing trans man and you have male privilege, it also means you have male disadvantages. If your girlfriend abuses you, you are not believed and there is nowhere to go. You are more likely to be stopped by the police. You are considered responsible for the financial welfare of your family. You are considered strong and invulnerable no matter what happens to you, and any sign of weakness means that you are less than a man, not because you are comparable to a woman, but because a man's worth is in his strength. The notion of "male privilege" is very simplistic, especially since it doesn't address the fact that every privilege a man has comes at a cost. If there is male privilege there is female privilege too, and it is similarly costly.

Moreover, when you tell a trans man that he is going to have male privilege when he starts to pass, it blinds him to many realities of the world. For example, when I first started transitioning I had no idea there were pimps and sexual predators who went exclusively after trans men who looked like normal guys. I was told that sort of thing only happened to women (cis or trans), and that if a predator were after me it would be because of my perceived womanhood, not because I was a trans man.

It's like some sort of bizarro-world nega-TERF ideology.

I think the reason these people hate trans men is because they hate men in general, which is the same reason TERFs hate trans women. The difference is who these people consider to be men.