I don't really see how his scatter-brained approach to login management is any less of a problem there, or on literally any other system that uses an email as a username or a password recovery mechanism.
I'm all for using a password manager. I also think it's not user-friendly for a single application to force one to use multiple email addresses. It's unnecessarily confusing and annoying.
Slack doesn't require you to use multiple email addresses.
I log into most of my slack teams with a single email address - to log into Slack, you need a unique (slack domain + email address) combination, not a unique email address.
I don't really see how his scatter-brained approach to login management is any less of a problem there, or on literally any other system that uses an email as a username or a password recovery mechanism.