I agree with that video. SJW Mary Sues are a bit different from standard ones. And that slight difference makes them worse somehow.
Honestly, I feel like Mary Sues are a tie for the worst character archetype, the other being the exact opposite--a villain with zero redeeming qualities. Then neither character is relatable and instead of developing an attachment, you just want to throw the book away.
A Mary Sue exists mainly as one thing: a self-insert wish fulfillment character. The thing these writers don't understand is that you can't just insert some random character into the story to live vicariously through. Even if you have an original story idea and it's not just some fanfic with the serial numbers filed off, having a character with no real flaws is going to kill the story. Look at something like Twilight. I almost guarantee you the biggest reason it was hated was the two main characters being Mary Sues. It doesn't make for good or relatable writing. It just makes for hot garbage. The fact that so many of the stories with Mary Sues, esp SJW ones, were written by women who are older than 13 is just plain sad.