The "etc" is pretty important here. You can log in using Reddit, and you can create a random throwaway Reddit account without filling in any other details (no email address or phone number required).
It used to be that reddit had a user creation screen that looked like you needed to input an email address, but you could actually just click "Next" to skip it.
The last time I had cause to make a reddit account, they no longer allowed this.
But it is true that at any time they could make using an email address or phone number mandatory, and then creating an Advent of Code account will be gated behind that.