I'm not sure I understand what you mean by this, especially since you go on to say that artist communes in the US qualify. Surely these artist communes exist within our centralized political and cultural system as well? These communes presumably refer crimes such as murder to the centralized authority, the police, rather than lynch mob.
I actually suspect that any significant regulation of Mastodon would in fact increase decentralization. Since the software itself is open source, it will be hard to regulate, if not impossible (is code speech?). That means regulators have to go after hosts of instances. Perhaps some instances will comply with regulation, but I suspect many will not. The largest of the non-complying instances will be shut down.
If the complying instances are seen as inferior to non-complying instances, instead of flocking to the complying instances, the users of the instances that were shut down should distribute themselves amongst the remaining non-compliant instances. Some of these instances will be outside of the centralized authority's jurisdiction (for example, after SESTA, many sex workers flocked to a mastodon instance that was specifically set up for them; this instance is hosted outside the jurisdiction of the US).