This is kind of the same answer as above. Our idea of what counts as “historically authentic” is very skewed by what texts survive and the contexts in which those texts were written. And, for that matter, by the perceptions and misperceptions of those texts that have percolated into popular culture. A lot of terms that seem incredibly modern actually go back centuries, while a lot of terms that seem very historical-ey are actually fairly modern.
Plus, once again, these are novels not documentaries. Characters often use deliberately anachronistic language to give the text a modern sensibility. Like the way Bridgerton uses classical covers of modern songs in its soundtrack.