>>534841902
Two things

One; The eternally relevant picture related. Specifically the bit about how Deltarune might have different rules.

Two; As far as I know, the concept of boss monsters has not been elaborated on in any meaningful way in Deltarune. Ralsei uses the Dreemurr fire and looks like a boss monster. Would you say that makes him one? I assume not, since Darkners and Dark World rules complicate things, and no specific conclusion is implied by his similarities. I don't think we can simply say that because Undertale's boss monsters are in Deltarune they share their Undertale functions as well.
That doesn't mean you're wrong, just that what your preferred viewpoint is most likely headcanon, at the moment.
The function of redefining boss monsters? I dunno. I bet it will depend on what the Light World or Asgore's chapter 5 appearance have in store for us. Though, in my opinion, boss monsters should just be left as an Undertale concept only tangentially relevant to Deltarune. They strike me as very much a trapping of Undertale's plot contrivances