This isn’t the case for these watches in the UK as well, as they only work on certain partner networks, but I’m unsure whether this is due to the GP claim or something else.
It's not like phones where pSIMs were relatively brand agnostic, were back to per-hardware BS.
I can imagine Apple being less inclined to allow arbitrary eSIMs on their watches, preferring to only support the case where the eSIM is guaranteed to support the "correct" presentation.