From what I understand, it seems that the current stance is that if the pokemon existed in Gen. 7, they'll retain that learnset. Likely has to do with the massive culling of moves Gen. 8 did, both in terms of what moves made into SwSh at all, and reducing what pokemon can learn from the moves that did make it in. For example, Vespiquen losing both Heal Order and Roost was a massive nerf to it in the main series, but letting it keep both those moves keeps it from being ruined here.