> a boundary is an ultimatum you're setting on someone else's behavior

No, it's not.

A boundary is something you're saying about your behavior. "If you use racist language at me, I will have to end this conversation."

And much, much worse than someone with "a minefield full of unnecessary boundaries" is someone who has boundaries they don't tell you about.

You should only set boundaries that are real boundaries for you, not just whims or arbitrary decisions. But if you do have boundaries—and everyone does; if you think you don't, then you just haven't had someone cross them (or haven't realized that's what happened when they did)—you must communicate them in contexts where there's a real chance of them being crossed.

To do otherwise is unfair to everyone else and to yourself.

I'm not sure how that example differs from an ultimatum.