What this is actually about: BlueSky is Jack Dorsey's new Twitter clone, it is eventually intended to be some sort of fediverse thing but it's not there yet and it's not the source of the fediverse gripes here. You can authenticate your BlueSky user as the owner of a given domain or subdomain by placing a certain file with a given content somewhere under that domain/subdomain. However that "somewhere" was just a location one of the devs at BlueSky chose, rather than somewhere relatively standardised, like under the ".well-known" path (which you might recognise from things like OpenID Connect where the configuration doc is located @ example.com/.well-known/openid-configuration). So one user exploited this and became the "owner" of that Amazon S3 domain by setting up a storage account on Amazon S3 and following BlueSky's setup instructions. That is the main story here - some non-Amazon rando is now officially the Amazon S3 guy on Bluesky.
The next part is that someone posted about it on this https://chaos.social Mastodon instance, which got overwhelmed, the owners decided to save their server by electing to return a 429 response for that specific post if users don't belong to chaos.social, and that is why people are upset about Mastodon.
Interesting story, but I'm not interested in Dorsey's version of Twitter 2.0 unless it actually allows you to signup[1] and brings something compelling that Twitter didn't and Mastodon doesn't.
[0] - game with an intricate story that does its damndest to not actually tell you. If you want to know the story you have to piece it together yourself by picking up dozens of items scattered throughout the game and reading all their descriptions. Or you can do what I did - watch a video on YouTube.
[1] - they're doing an open beta and letting a little trickle of users on, who post about it on their Twitter/Mastodon/whatever. Feels a bit deliberate, like they're trying to build anticipation and frankly I detest little manipulative things like that so I'm out