dots-don't-matter isn't sufficient to fix this issue - any other canonicalization issues would surfacea as the same problem. That might include casing, unicode issues, the `+irrelevantsuffix` stuff, and probably more, including perhaps any kind of aliases in the hostname.
The idea that you can tell whether two accounts are the same is questionable; netflix should not be relying on it.
Furthermore, the hack is possibly much less serious (but nonetheless conceivable) even without any canonicalization issues whatsoever. Apparently, a user can trick netflix into sending email to arbitrary email addresses. Sure, this is likely not to be an issue if the recipient doesn't have netflix in the first place - but if you try 1000 times (which spammers and phishers are wont to do) - you may well happen to hit a person that'll pay nonetheless - perhaps they forgot which email they used for netflix in the first place, or perhaps they share an email account, or perhaps the real owner is dead/on vacation/otherwise unavailable and somebody else is tending the account and thus more gullible than usual.
Netflix should never ask for money from random strangers without making 100% clear what the context of that request is.
(If indeed this article is accurate, because I wouldn't rule out the chance that the author made some oversight earlier).