IDK, I think it’s difficult to argue that GameFreak liked the possibilities ILCA brought to the table when they also didn’t let them apply any of those possibilities to their game lol. The underground was a better execution of the Wild Area but a worse execution than Wild Pokémon in SV, and I wouldn’t be at all surprised if that was both requested by and worked on alongside GameFreak as a technical exercise for them.
I’m sure that this type of thing has happened before — reportedly XD: Gales of Darkness exists because Genius Sonority wanted to make a sequel, as opposed to GameFreak or TPCi wanting one — but I’m also sure that those games are considered spin-offs internally. If ILCA requested a spin-off then sure, maybe you’re right, but it’s a risky manoeuvre considering BDSP had at least the worst reception of any mainline Pokémon games since BW (SwSh’s trees don’t count, those games were still pretty well-liked). When BW got the reception it got, the sequel addressed essentially every concern from the originals (bringing back old mons, a less obviously linear region, etc. etc., the only thing they didn’t change back was infinite use TMs lol). It makes sense that GameFreak trusts themself to do that, but why would they trust ILCA to do something completely different from the only project they’ve ever done? Even if ILCA did request it?
And even if it is a spin-off, ILCA’s name is heavily tied to BDSP among fans. They have to know whatever spin-off they come up with will be met with a great deal of skepticism. It doesn’t make business sense, really.
Even the art style was extremely divisive. Love hate to an extent, but mainly hate that I saw lol. I can’t think of a spin-off that weird chibi dollhouse aesthetic would be the optimal art style for besides Pokémon Rumble. And if it’s Pokémon Rumble then I take it all back, those games seem pretty easy to make well. Of course that was still messed up at some point by a freeplay model but I digress.