It was an event run by a ganker group called Borann Boys or BRNN for short. They have either one of their members or a friend running an alternate (Free Epic Store Account) with the Elite Not So Dangerous Trainer in Horizons to cause Thargoids to spawn. You can also see the CMDR in question is flying a DBX with a Thargoid Pulse Wave module equipped, which is impossible for a player to use without some sort of an exploit or cheat running.
When called out on this, CMDR Quidd of BRNN (You can see him winged with the cheating CMDR in the clips in the OP), responded: One of our friends farmed 1000 Thargoid Probes and dropped them in open space by the station which triggered a Thargoid incursion into the system. Which IMHO is total rubbish. They are using an exorbitant number in hopes people will actually be discouraged from putting this claim to a test and hoping people wouldn't care that they were actually running some sort of exploit/cheat.
If you follow this YouTube Link it will take you to a video that was made by one of the BRNN members (Aka-Howitzer) which showcases edited clips of the event from their perspective. See this link for a copy of OP's video of another POV that also showcases the CMDR (Dev Test 1) that was actually running the Thargoid wave module on the DBX.