You do realize that it has a 7.2 second cooldown, and you haven't looked at his video of 1300 AD vs 1600 AP. Although Vandiril is a sensationalist who creates scenarios which never happen, look at what happens during his AP Irelia kill. He murders 4 of 5 in 1 skill, and the last in his first combo.... and it happens fast. The R has double hits: initial hit + dagger wall hit. And it can hit multiple targets with both hits, and has a 70% AP ratio.
AP is actually more viable than AD at erasing teams with her new ratios/skills, and 3 of 4 skills are aoes with disarms/stuns. It's not garbage. You can build her
and have enough damage on your Q to reset it on each use, and it'll also pump all her other abilities.
For example, this build has a base 405 AP, multiplied to 567 AP, and that doesn't account for any AP she gets from runes (possibly boosting up to about 760 AP or more) Also, the new Ginsoo's is a % boost to AP, resulting in a charged bonus of...664 AP and 203 AD.
On her incoming parameters:
R: 864.9 Initial Hit
739.8 Secondary hit
E: 837.6 damage
W 1087.6 Damage.
All these are aoes, on relatively low cooldowns, other than the R.
That's a total caster damage of 3,529.9 Premitigation damage before even considering her Q's to a large aoe, with no risk in a team fight. And it doesn't even include her Runes.
Here's what I'm looking at:
Sorcery Primary: Either Phase Rush or Comet (Comet is also an aoe, and Irelia has a trap to make it hit)
Ultimate Hat
Celerity/Transcendence
Gathering Storm
Secondary varying based on team.
That's going to give a LOT more AP, which means built as an on hit mage, she's going to do over 4000 damage just with the caster abilities, and have 3 seconds of complete immunity per 7.2 seconds cooldown.