if he ACTUALLY did any such thing the FAA would have issued a notice requiring aircraft WiFi
You obviously didn't read the search warrant.
First, it states that in previous interviews (in Feb, and I'll bet the FBI has audio records to support that), he had described connecting to the network using Ethernet connected to a "Seat Electronic Box" ("SEB") which is mounted under the seats. So, WiFi has nothing to do with it. In the same interview, he said he understood the legal ramifications and would not access airplane networks.
The warrant goes on to state that the FBI inspected the SEBs around the seat he occupied on his 4/15 Denver to Chicago leg, and found signs of damage and tampering.
That, along with his history and the tweet regarding being on the flight and suggesting he could tamper with the flight systems seems to me to be reasonable grounds for a warrant.
And, I hope he's prosecuted. Also in the Feb. interview, he admitted actually tampering with flight control systems. It's one thing to find a vulnerability and try to get it addressed. It's quite another to actually make use of that vulnerability during a flight, placing the public at risk.
This file will self-destruct in five minutes.