That kind of list needs pretty good moderation to avoid that. Also, it's not really that effective considering offenders can easily move servers.
Hell, it would be all to easy for adversarial group to just server hop and get a bunch of sites blocked purely because the mods of those sites are not on 24/7... or just allow for free speech
Nah. De-federating is manual. Such an adversarial group would need to join up, start spamming, and keep on spamming long enough for their instance to be added to a blacklist, and for other admins to pull that blacklist in, without the admin ever acting in the meantime.
It's not like the fediverse is that big, either - people talk to each other.
Am new admin for a friend-only instance; currently blocking seems to be done via user reports + checking in on the #Fediblock hashtag. It's all manual, and repeated admin work needs to be done per-instance, which is why the article says it's inefficient but a good trade-off (so you can choose the server you align with best).