Managers often feign cluelessness because what else can they do? Tell you they submitted you for layoffs? Tell you they knew for weeks and said nothing? There's really no upside option here.
I have no doubt that sometimes managers really don't know, but I'd wager that most who say they didn't know probably did.
I'd wager they didn't.
Lowest person who generally will know will usually be a senior director. Sometimes director. Sometimes VP.
Google is pretty careful about this. While it's true sometimes those people share stuff, it's definitely not en masse, and you can get yourself in significant trouble if you share it when you shouldn't, so it's less times than you think.
At most other places i worked it was different.
From my personal experience, front line managers are often not privy to layoffs.