I think from this person's posts that they are gender critical and agree with you that the trans man is female. They're just making the point that there are a few specific limited scenarios in which pointing this out in a particular way (unnecessarily / achieving no purpose and with the intention to cause distress), motivated by an intense dislike for that person or their characteristics, could be construed as hateful. Which I think as fair. It's not the same thing as saying it's always hateful to state the biological reality of sex - that's a false equivalence indeed.
I can think of another example. A trans man is dying, you approach their death bed and say "you're not and you'll never be a man, human beings can't change sex" - I think that would be hateful. Obviously, it's a bit of a stupid example because GC feminists aren't doing this, they're just pointing out reality because it's pertinent to their feminism.
In a way these random specific examples help to demonstrate that generally speaking, it isn't hateful to point out that human beings cannot change sex.
For the pedantic among us, it can be difficult to resist the gauntlet being thrown down (eg "try to think of an instance where staying the biological reality of sex is hateful"), that's all. *I actually personally disagree with the bus scenario, I don't think that meets the threshold for hateful! But it's possible to discuss the matter in good faith.