The thing is, taking the other side (that it is not RNG only) seem to create a worse case to prove, and almost certainly carries the burden of proof.
How stupid do some of these sound to you:
"Shinies favor casuals"
"Shinies are locked to the player and each player has better odds with different species"
"Only 2 people can get a shiny in each raid group, so split your teams!"
It is a very sensible assumption to assume it is indeed RNG, unless there is something so obvious that is missing which TSR/this site would have picked up.