In very essence, when he says he might be the most powerful vampire in existence, he's not lying.
He's exaggerating quite a bit, though; as far as 5e vampire spawn go, Astarion is kind of a wimp. No regeneration, no damage resistance, no spider climb, no claw attack, his bite doesn't have the extra necrotic damage, and if I'm reading the entry right, he's missing a natural armor bonus. Walking in sunlight and entering houses is probably great fun for him, but he's lost a lot of power. Of course, he also seems to be free from Cazador's control, which I'm sure he thinks is worth the nerf.
It's interesting that healing spells work on him. Maybe that's just not implemented yet, though.