Do you know if they actually don't bother? The article author makes it sound like tracking companies do in fact normalize gmail addresses. I suppose my custom ___domain addresses wouldn't be too hard to normalize either if you have a way to take a ___domain and get a probability for how likely it is to be a personal ___domain. It provides a little extra friction at least, I guess.
Whoever with more than two brain cells would notice and strip the suffix out, but they usually don't bother with a database leak.