"Armor's better protection against Master Yi than it's ever been."

That is the opposite of a problem.

Riot's actually endorsing the fact that Yi did Physical, Magical, and True damage so one defensive stat isn't enough to defend from his attacks even when he's building tank items. Now that Bloodrazer isn't magical, they're pretending that's an issue despite the true damage he does.

And since there's nothing mechanical you can do to outplay his completely targeted kit, this is a way of saying they don't want him to have counterplay.

Side note: Perfect time to use those three Yi skins I was mystery gifted.