To me, the difference is positive vs negative acts.

An ultimatum says “you must do X or else I will do Y”

A boundary says “you must refrain from X or else I will do Y”.

I guess my question is: What's wrong with an ultimatum over things that are actually egregious enough to need a hard boundary? It seems like you're stuck on the word "ultimatum", as if there's nothing that could possibly be acceptable to give an ultimatum over.

I mean...I'm a pretty easygoing guy overall. My boundaries are things like, "If you come up and scream in my face, I'll tell you to sod off." "If you punch me, I'll probably shove you back." Reasonable boundaries for other people might be "if you grab my butt, I will report you to HR", or "if you ask me to work unpaid overtime, I will refuse (and report you to HR/the NLRB)".

It seems like you think "people setting boundaries" looks like telling your coworkers things like "never ever speak to me in anything but the most respectful tones" or "if you ask me about my personal life, even the tiniest bit, I will call the police". Except in extremely unusual circumstances, "boundaries" like that are actually people being abusive of their coworkers.