I mean, sure they can be entirely confident that I'm the Facebook user who signed up for that account, and so in that sense they know exactly who I am.

But in another very real sense they've got no idea who that is. It would suit them very well to be able to reliably tie it to other information (hence all the tracking pixels and so on) but the Container prevents that.

I mean, one of my Facebook friends is named say "Norman Le Plum". I'm very confident that isn't what it says on his birth certificate, and indeed when his friend request arrived I actually ignored it until I found someone out of band to tell me who "Norman" was, but in a sense Facebook know exactly who Norman is, he's a disembodied red skull who is still really into skateboarding and Steamed Hams.

What use that is,isn't clear, and presumably one day advertisers might conclude the answer is "No use whatsoever" and Facebook will go out of business. Meanwhile I read funny Steamed Hams variants, people complain about their jobs, and while I'd rather it didn't exist at all, if it must exist at least it's trapped in a little box where it can't taint everything else.

Now Google probably knows way too much about me, but that's quite a different problem.

I wouldn't count on a fake name being any kind of problem for facebook assuming they're actively using the profile. Not providing them any data at all won't spare you, but if you're using the account they can easily analyze photos and comments (including those on other people's profiles), use facial recognition, use friend/activity patterns, match IP addresses/browsers (including any instances where the same IP address/browser was used to sign into non-facebook services found in records purchased by facebook from data brokers), and if he ever uses his phone or chrome to look at facebook there's a handy unique ID sent to facebook as well which can be matched with countless other recorded activities.

Facebook devotes a huge amount of time and money to collecting data and using it to associate people to a real identity to the extent that even people who never signed up for an account at all have hidden profiles created for them by facebook which contain the intimate details of their life including what they buy at the grocery store.

As far as I can tell, a container won't protect your identity but it will limit the amount of information they have on your browsing history (unless your ISP decides to sell them that information or they obtain some of it from a 3rd party data broker)

Older accounts may have fake names but newer ones require identification documents, even selfie videos to prove you are a human. It wouldn't surprise me if they start combing through older accounts eventually.