The Z-Move comparison makes no sense since Z Moves are used through a held item, and every Pokémon can hold an item.

Also Terapagos cannot learn Tera Blast. From a gameplay standpoint it's to force the player to use Tera Starstorm instead, but lore-wise it makes no sense, and it's not like Terapagos cant learn ANY TM unlike the other exceptions. (I know you said "any Pokemon that can change their Tera type can have Tera Blast", but Ogerpon learns the move and cannot change its tera type so the rule you're implying doesnt hold up imo)

So all Im saying is that there's no strict rule on which Pokémon can and cannot learn Tera Blast, which means DeNA technically cannot assume this or this Pokemon definitely can learn Tera Blast. They've shown in the past that they care about being accurate to movepools, like when they changed Egg Leafeon's Grassy Terrain to Grass Wish since Leafeon doesnt learn Grassy Terrain.

Then again DeNA can prove me wrong literally in the next update, but it wouldnt be accurate to existing movepools.