Too many enemies to guess at this point

It's not a credible thesis, at least not given the info we have. Tier 1 cyber enemies would have done something more subtle (so as not to trigger a kinetic warfare response or a response in kind) or something so obvious that they announce it with a declaration of war.

It's far more likely that the creaky old civilian radar infra has problems or some old single point of failure (maybe an old mainframe) has/had downtime. There are 3 widely rumored causes of this FAA outage (with seemingly no evidence) related to projects that the Feds were already working on making: (1) an AI air traffic controller assistant, (2) migration to something newer than 1970s floppy disk tech, (3) migration away from radar to GPS for controllers).

All of these are more likely than a foreign cyber team attacking FAA systems in this way.