I think it's just very messed up, ultimately it doesn't work against the real nasty people Reddit claims to be going up against because those people have bot armies that monitor their astroturf accounts so they know when the shadowbans happen and dump the account to move on to the next ones. No this system disproportionately affects the people who aren't expecting it and probably don't even deserve it.

Also for braindead spammers it's actually a terrible strategy because spammers' purpose is both to annoy users and chew through your resources, even if they are shadowbanned and uploading multiple gigabytes of white noise they aren't annoying people but they are chewing through bandwidth and CDN storage. IMO that's not feasible long term, and wouldn't even be initially feasible for most Fediverse services, hence why most basically just don't do it.