I don't find this a convincing rationale. I already had to jump through hoops with an authentication code to make my iPad "trust" my Mac before it appeared at all in Side Car.

Now that it does appear, I get a pop-up when clicking on the iPad income, saying I can't use it because the account doesn't have 2FA set up. Nowhere in the advertising for the device was there a disclaimer that a legacy phone number was required to use the features.