Not illegal. But most people without resources to fight off the thinly veiled, strongly worded legal letters would probably fold under this light pressure.
Interesting, but I suppose it’s not surprising to see clients still holding references to old/defunct trackers. Those peers this person discovered once the tracker was resurrected are more than likely to be seed boxes. Maybe a few real clients if they found an old .torrent link and have left it open.
Thanks to DHT (trackerless peering), trackers have become mostly defunct.