If we go by Dio Vento's Lets Go 151 Mod as you used in your example, the thing is all 151 pokemon models already existed within both games; It is not as if he tried adding new pokemon from other generations which is essentially what you're asking about for Sword & Shield.
If we use older generations rom hacks as an example, despite how active that scene was (particularly 3d gen hacking) they could never manage to exceed the number of pokemon within the default games; This is why rom hacks that try to add future generation pokemon have to replace an old one first (due to limitations within the game engine most likely or lack of memory possibly, I am unsure).
If there are leftover placeholders for the remaining pokemon, that is wonderful and all, but that does not mean other issues wont come up down the road that may render the task impossible; This would probably be a game engine related limitation in which case we'd be SOL without the game's source code.
Whether or not mods of this scope would be or will become possible for Sword & Shield, will ultimately come down to technical limitations 1st and Burn out from volunteers 2nd; Even if such a mod is possible, if we go by precedent I would not get my hopes up (or at least not expect anything for awhile).