As mentioned already, this no-fly zone was promulgated long before the December incident. The statutory instrument (formal legislation) is
here and the NATS circular is
here. As mentioned already, ATC-controlled Heathrow (and Northolt) movements are exempted.
As also mentioned already, the area is class D airspace which requires ATC clearance. But the royals are often targeted by fantasists, 'activists', pranksters, etc. Mostly the incidents don't get wide publicity. There were at least two earlier in 2021, besides the knife-wielder at Christmas. I don't think anyone would be surprised if some idiots decided to try flying an aircraft – manned or unmanned, big or small – around Windsor to try to make some point or other. They certainly wouldn't be deterred by regulations. I don't know, but I'd guess that the reason for creating a no-fly zone is to give the authorities more flexibility in dealing with these people. Maybe a breach of a no-fly zone can be dealt with more expeditiously than mere unauthorised flying in class D airspace.