Writing Problem: Modern Jargon

I was recently quite disappointed with a new M/M romance by an author that I generally trust. She has written a number of books I enjoy and even admire. 

The book was ostensibly historical--but was, actually, an alternate world, which used modern jargon. 

However approved, terms like "gender" and "they," as a reference to an individual who ducks biological designations, are jargon. 

The result: the book felt incredibly dated. I gave up.

It was a curious experience since I've read other books that use both terms and didn't come away from my reading with the same reaction. I've determined that the difference comes down to something that Jane Austen--to use a worthy example--manages in her books: she uses references, vocabulary, and very occasionally slang in her novels. Yet, they don't feel dated.

The references and vocabulary and slang are appropriate to her characters. Similar to Stephen King's cussing blue-collar Mainers, the dialog doesn't feel forced. It is how people talk (in comparison, I've read literary pieces full of swear words that felt completely hammed-up). 

In one book, where critical thinking jargon such as "gender" and "they" appear, the alternate world is actually our world--post-COVID (which event is cleverly used)--in the omegaverse (a world where men bear children). The terms are used by characters who would in fact use them. In context, they point to a complex political and media reaction to an unexpected bonding outcome. 

In another book, the terms are actually not used though the concepts are addressed. Since the concepts are addressed from within the character's historical perspectives, they come across as natural reflections rather than belabored, pushy preaching. 

The same differences occur in Star Trek: TOS. The episodes that take place within the Star Trek universe, even though they reflect modern concerns, come across as natural extensions of that milieu and those characters. 

But the episodes that deliberately try to comment on the modern world come across as dated--even when amusing.

"Charlie X" works. "The Way to Eden" is painful.

"Turnabout Intruder" raises questions while remaining truthful to its premise. "Wolf in the Fold" (which I personally like) fails. 

"Space Seed" is excellent. "Return to Yesterday" is fun and less painful to watch than "Assignment: Earth" but still dated.  

"The City On the Edge of Forever" is fantastic, despite referencing peace movements in America--contemporary to the writing of the episode. 

When story takes precedence, story adequately carries theme and perspective. When it doesn't, the entire edifice falls apart.