You claim anecdotally that a lesbian wouldn't say they scissor. Pointing out that they do say it suffices. Your experience doesn't encompass all lesbians of every generation, so we can refer to LGBT+ publications, direct anecdotes, history, & scholarly research. We have direct anecdotes from a podcast & affirming comments. The wikipedia article even calls out exactly what you're doing & cites scholarly research. It explains with references that it is or was common for some tribadists not to recognize that word (or variants) & use scissoring more broadly to describe their activity.

Elsewhere, you add that a specific sense of scissoring (that the comic didn't specify) of rubbing genitals together isn't real. The cited references have some interesting quotes. In a Hite report

Sex with a woman for me has involved kissing, feeling one another completely, and basically humping – pressing mound of Venus against mound of Venus or each other’s leg.

Another cited reference explains tribade historically meant a woman taking the role of penetrator (with dildo or clitoris) before describing Anne Lister's diary where she mentions genital to genital contact in her tribadism.

Most of their sexual activity seems to involve Anne touching Mrs. Barlow's "queer" (as she calls the female genitals) and tribadically rubbing on her.

Anne manipulates Mrs. Barlow digitally but would prefer full-body tribadism, queer to queer, as she says elsewhere.

So, they do say they do that, too, even ages ago.

As for the ngram viewer, it's not essential, however

• it's true the corpora don't reflect general language & culture, only that in a corpora including over 40 university libraries around the world
• similar trend as corpus without scientific literature indicates that's not a source of bias
• we're only reviewing recent texts with high quality OCR, so OCR issues don't apply: OCR improved, not poorly printed archaic texts, tribbing doesn't contain mistakable characters like long s ſ so OCR error should be uniform, not Chinese or a language difficult for OCR.

It indicates tribbing hasn't historically appeared in print much.