The art of rhetoric and human communication involves knowing your audience, and when using idioms and metaphors, you choose those which are shared in common to be immediately recognizable and impactful.

When I was 19, I had a psychologist who employed a lot of baseball idioms. "We'll touch base next week..." and although I am not a baseball fan, my father is, and we're all Americans, and so the baseball references were well-understood by his audience of one.

When an LLM starts going off about a "tractable beachhead" that behavior indicates that it's unaware of its audience, and also ignorant of how to put together a human idiom. That's exciting that emergent behavior is coining new idioms that may be applicable to some situation, and also explainable by dudes on Hacker News, but that wasn't the point of the comment.

The point is that the LLM's audience wasn't a veteran of D-Day in WWII, and even with an understanding of what those words mean individually, neither he nor I recognized "tractable beachhead" as a meaningful idiom for speakers of standard English.