Kinda on LoA
This actually is a consequence of the way NatDex is defined: every move works like it did the last gen it was introduced.Why is Smeargle able to learn Double Iron Bash?
Well, and every other Gen 8/9 move that isn't Life Dew or Body Press, since those can be learned in BDSP which Smeargle is in fact in.
It's not fair that Smeargle gets to have moves that it's never been able to learn in any game just because "that's how we think that Sketch is going to work".
Not decreasing Heal Order's PP to 8 makes sense, because we don't know if it'll be nerfed if it will even exist after this point.
Allowing Smeargle to have these moves doesn't make sense, because if we go by that logic then Tera Blast, as a near-universal TM, should be accessible to all pokemon that are capable of learning near-universal TMs.
Smeargle is already unable to learn Chatter, and I believe that in situations where we have no information on if a move is sketchable or not, it shouldn't be just like in situations where we aren't sure if a move will become learnable or not.
The only condition you have to meet for sketch to be able to copy a move is being in the same battle than a pokemon that learns that move, including in multiplayer battles (except in gen2). In National Dex, Smeargle faces Pokemon using these moves. Only three moves are not sketchable (Chatter, Sketch, Struggle), so not giving Smeargle every gen8+ moves would be a very strong assumption on how Sketch would work in gens where it doesn't exist.
In a nutshell, Smeargle learns every move for the same reason Heal Order kept its 16 PPs. We can't assume changes for nuked moves, and that includes Sketch.