Damon, with all due respect … that part of that standard is a result of someone hacking something together without thinking it through and then just selling it to a committee that did not bother either.
A long time ago, when I was in the alpha-testing team on BibbleLabs and they went from their homemade .bib sidecars to xmp I was one who initially fought them on the filename.raw.xmp idea. But that changed the moment I shot a job in RAW+JPG and was able to edit them side-by-side as I pleased while maintaining complete flexibility. I was happy that they ignored this idiotic piece of the standard. Today those 15 year old xmps open happily in darktable with all the metadata present. Interoperability with any Adobe tools has been a hassle, as always. So yes … the filename.raw decision by hack plus committee was a bad one.
Of course I do not know what the exact story behind filename.xmp is, but the result is clear: use Adobe or die. Other members of this forum have already written thorough explanations, but since your argument trails around “it’s a standard” I have no hope of convincing you otherwise … I’ll just leave this link here: