It's true that the concept didn't exist, but we can theorize about whether people had some of the same feelings and relationships that we (queer people) have. I think that's what people are saying when they say that someone was "gay" (or "trans" for that matter--it gets a bit messy because some of the same people are claimed as gay and trans, but this is to some extent just a result of not being able to tell 100+ years on why someone acted the way they did).
You can believe in an innate sexual orientation and I guess I do. I think people can act based on their innate sexual orientation without identifying it that way. So I can try to figure out if historical figures might have the feelings and experiences that would add up to that.
It's kind of like people with autism theorizing about whether famous historical figures like Einstein were autistic, or about whether the idea of changelings came from autistic children. Obviously changelings weren't perceived as kids with disabilities, they were perceived as monsters. But autism is real, and if they were autistic, that's what they really were; they shared an innate quality with modern people who are autistic. They weren't really changelings just because they were thought of that way.
I mean, we're stuck in our time too and maybe our concepts are going to be proven wrong, or at least become obsolete, but I think we can try to find history that involves the kind of people who we describe using those concepts. I don't think it's inaccurate to say someone was gay because being gay doesn't necessarily require calling yourself gay. And the thing is queer and trans and autistic people are always being accused of being some new invention, and told that we have no history; that's why we're kind of starving for it and that's why we claim people. So making a big deal out of the fact that "they didn't identify that way" actually seems kind of cruel. This is important.