It does, actually, but there's a bug in the spell description that makes it look like he's capped at 20 damage - while in fact he does Magic+4 (before boost from removed gems).
When was it ever x4? it was X3 ever since he came out. It sounds like you claim there was a nerf, but no such thing ever happened.
TDS is plenty strong. He explodes most of the board, deals AOE damage (comparable to other legendaries when in a full dragon team) and gain souls. Compare his spell to that of Gorgotha or Behemoth, and you'll see there's nothing to complain about.