Adding a phone number to your Google account can make it less secure
Well of course it makes your account less secure. It's another attack vector. As shown in the post, Google doesn't say add a phone number "to make your account more secure", it says "so you don't get locked out". Intuitively, making it more difficult to get locked out of your own account would likely make it easier for someone else "not to be locked out" of your account.