You can run your own server, on your own hardware, make your own account, own every step of the stack, and can comment on there.
It's not just 3rd-party login, it's YOUR-party login.
You still have to make an account on a totally unrelated website. It's the shittiest experience one can think of.
But your excuse is "Hey actually you can also setup the whole application yourself before making the account to post a blog comment on an unrelated website, so that's actually super awesome!".
Absolutely ridiculous, and if any company you didn't like were to do something similar you would screech.
Even back before 2010, almost all blogging platforms had an option that would let the owner require log in to comment.
And it was fairly common, because of spam.