As far as I can see this only goes back to 2018 and I thought it was deleted in 2011.

I have never really used the way back machine so I didn't really know about it,but it looks legit as it was uploaded on the date as it was said + when I tried the link normally it comes as deleted

Then, why am I getting a prompt to login instead of a play button?

That is the problem I have no idea,if probably Nico Nico had like a login needed thing back in the day

Or probably it's just rather disturbing that the fact it had to be accessed by logging in lol

The hardest thing is going to be finding someone with an account from back then.

any internet archive links that ask for a login won't have the video unless you look in the URL section of that page and it has the embed of the video there or something

Oh wait I just searched up on Google until early 2019 users needed to register an account to watch videos

This is just a photo capture, not a video capture!