This is demonstrably untrue. At least one gay porn star identifies as a bottom.
Obviously, NSFW things follow.
Armond Rizzo is a gay porn star of some repute (254k Twitter followers), and the top page or two of results I got for a Google search for ~gay porn star self-identified bottom
.
According to an interview with "Gay Demon" (explicit images present on site, in addition to explicit sexual language):
You're quite clearly more bottom. In most scenes you're 100% bottom and I've seen you flip fuck maybe a couple of times. What is the draw for bottoming for you?
Bottoming for me comes so easy. I enjoy it and love it so much and I am 100% committed to taking what ever challenge I get. So it really does not take much for me to bottom. But to top, well I guess those times I was forced to do it, although I didn't mind. I love being pushed to try new things and I love to try to embody a new character. So to flip the switch and be top is pretty exciting too!
[...]
Time to help me out here. I admire that hole of yours so much. It's super stretchy and you can get really wide. How did you prepare your hole for such big dicks like Noah Donovan's?
I honestly do not know how how I do it lol. I guess being and athlete and a true bottom that loves getting fucked. I just control my breathing and relax my hole and be a good bottom that just takes it. Like I always say, "Don't call yourself a power bottom if you're going to bitch and complain about how much it hurts, if you're going to do that than you should've became a top so shut up and take it!" lol
And there's also a video interview (I haven't watched) titled "Armond Rizzo Interview on being Porn's Famous Bottom" that can be viewed on Pornhub (among other places). So, it seems that he identifies as a bottom, and that identification is played up in the industry.
Being a heterosexual man myself, I can't provide any insight on how well-known this particular gay porn star is, or the prevalence of gay porn stars who identify as bottoms, but there is at least one gay porn star who publicly identifies as a bottom, and uses that identification as a (the?) major component in his projected image. So at the very least, that disproves the absolute statement "gay porn stars never identify as a bottom" with a refutation by counter-example.