If you are not familiar with /r/place: the idea is that every user gets to place pixels on a canvas. After placing a pixel, there is a 1 minute cool-down before you are able to place the next pixel.
To log-in, you fill-in your instance's name and username, and you are sent a code as a private message which you then use to authenticate. So the method of authentication does not expose your username and password combination to a third party.
I find it cool how third-party events like this one can be integrated into the decentralized Lemmyverse without requiring active input from instance admins.
I don't like to be a cynic, and if those pixels really were placed there by a very organized community of fans of my little pony I think that's actually very cool. But my suspicion is that someone used a bot to create that, and I'm not sure that they did it because they are fans of the cartoon. I'd love to see evidence of my suspicion being wrong though!
Since I wrote that, I looked more into it and I no longer think that it is a bot. I think think that they really are some very dedicated My Little Pony fans. They did move super quickly so I think they are using some strategies such as opening a lot of different accounts in multiple container tabs (which is not against the rules). I now think that they might actually like the show and are doing this in good faith, and in that case I was being too cynical in my above comment. Good for them!
I know of some people from the Phoenotopia fandom that have been trying to organize painting pixel art of the main character Gail onto /r/place. Gonna go take their templates and try to do this on Canvas!