Hi! I'm a Classics professor who works on ancient sexuality. My expertise is pretty limited to the ancient world and scholarship thereon, which means I can't speak specifically to 19th century doctors. But the word "lesbian" is from Sappho's home island, Lesbos, and sapphic obviously comes from her name so yes, those are words referring to women who have sex with women.

But let's talk about those historians. When you talk about historians thinking that Sappho and the women were "just friends," there's a couple of things you might be referring to. First, there's the hugely discredited (but still oft-repeated) theory of "Sappho Schoolmistress," in which Sappho was interpreted as a lovesick spinster who taught girls about love before marriage but never actually had sex with them. That theory mainly came from the desire to "save" Sappho from being a lesbian, plus a false parallelism with Athenian pederasty (where older men were paired with boys / teenagers in a supposedly educational manner, if we listen to Plato). But like I said, that theory is based on pretty much nothing. Holt Parker, "Sappho Schoolmistress" is the go-to article debunking that theory.

But the theory has still hung on in other ways, namely in seeing Sappho as the leader of some sort of "circle," poetic or religious, and writing poetry about the girls in her circle. This may or may not be coupled with the assumption that she wasn't actually sleeping with them. Here again, going back to Parker's article, there isn't actually great evidence that Sappho's poetry is written about "girls," that is to say, children or teenagers or anybody significantly younger. It's equally possible she's writing about age mates--"women." But that's a whole other issue. In the end, we just don't know the performance context for Sappho's poetry.

Or you might be referring to the idea of a fictional persona, namely the idea that just because a poet writes about doing something doesn't mean that they do. And sure - Sappho writes about her love for women, but that doesn't necessarily mean she was in relationships with them. There's really only one fragment that could refer to physical intimacy (Frag 94, which mentions something about fulfilling your longing on a soft bed, and the previous lines are written to a woman). But again, even if she wrote a sexy poem about sex, that may or may not say anything about her life.

Or you might be referring to the very common statement in scholarship today about how Sappho wasn't a lesbian at all. I talked about that once before, but basically, the term "lesbian" is very modern and there isn't an ancient Greek term for it that we know of (for Sappho's time and place). There's a view, based on Foucault, that "sexuality" as a form of personal identity didn't exist in the ancient world, so Sappho wouldn't have identified herself as Someone who Sleeps with Ladies even if she did. Sandra Boehringer's recent book in French is really interesting on Sappho; she points out that ancient authors view Sappho just as a love poet, not as a ["lesbian"] love poet; they don't seem to comment on or be freaked out by her writing to women. Nor does Sappho in her poetry make a big deal about it; she just writes poetry to women.

I sort of scatter shot this because I wasn't 100% sure which historians you were talking about but I hope this helped!