Email people can readily have multiple of, phone stuff can similarly be spoofed (theres sites so people can get numbers to get phone verification done without using their own phone number for example), and physical home address is both hard to verify and it's often the case multiple people play from the same household anyway.
If they wanted to stop smurfs, step one would simply be to make smurfing against the rules, and then move on figuring out actual enforcement of it and prevention, but they're not anti smurf in most situations (clash being the main exception) and smurfs aren't necessarily a big picture problem anyway. (They'll either only affect a relatively small number of games before reaching an appropriate mmr, or they're either not playing often [and hence not affecting many people] or actively inting/griefing in which case they can be punished for that regardless of their smurf status.)