I mean riot wants shorter games.
Its his job. He is almost full build with 3 mountain drakes. The rest of his kit is garbage.
That's kinda crazy, but it's also a full build 700 AP Ziggs with Lich Bane and Passive proc in one auto (and 3 mountain drakes).
"Ziggs's basic attacks periodically deal 20 - 160 (based on level) (+ 30 / 40 / 50% AP) bonus magic damage, doubled against enemy structures."
2 (160 + (.5 * 701)) = 1021 magic damage
"Spellblade: After using an ability, your next basic attack (on-hit) deals 75% base AD (+ 50% AP) bonus magic damage"
(107 * .75) + (.5 * 701) = 430.75 magic damage
"When basic attacking a turret, the damage is based on the base attack damage added to either the bonus attack damage or 50% of a champion's ability power, depending on which one is higher."
107 + (.5 * 701) = 457.5 mixed damage?
"Earthen Arms" grants 10 / 20 / 30% additional damage as true damage to Epic Monsters and turrets.
1021 + 430.75 + 457.5 = (1909.25 * .3) = 572.775 true damage
In the clip it says 1781 Magic damage and 572 true damage. Obviously my numbers are a little off cause I don't quite know interactions perfectly. I think the Armor/MR on tower should reduce the magic damage portion by more than 128, but even just my rough calculations show that shouldn't be too shocking.
Now, should they do that, IMO no, but Riot wanted shorter games and I think a sheen + passive tower destroying auto + tower execute ability is going to do that regardless of how much Health + Armor or MR you give a tower. And, to be fair, that's one champion that can do it and basically his whole champion design is sieging towers.