I would think that generally a straight guy would not do it as to avoid girls developing such doubts. But he could indeed be a passionate ally that does it for their close friend or family member.
I get the impression that this filter is a fad, it has no bearing on the posters sexual orientation. If someone is gay he will say so. Otherwise he’s either straight or closeted.
Everyone in Taiwan knows what the rainbow means, just as people do in the West.
The rainbow has no other cultural meaning here if that’s what you’re asking. If that is the filter he chose, then it most likely signals something related to LGBTQ whether he is gay or an ally.
My Taiwanese brother-in-law did that with his gf at the time to support his brother as an ally. You never know these days. There are a lot of straight guys out there that are vocal as asked. You’d really have to ask to know for sure.
Loads of girls here do it to signify they’re an ally. Not so many guys. Don’t think I ever have, I just use this type of thing. In general if I see a guy use a filter I’d presume they’re gay.