It does not have to be a 'secret cabal', just a group of like-minded individuals soft-coordinating their responses. JournoList [0] has existed for three years before being made public. When exposure forced JournoList to close, the response from the members was immediately to create another group like it [1]. What also telling is that the members don't think of themselves being a part of 'secret cabal', which allows for the double-think of coordinating responses while being sincerely sure they do no such thing [ibid.].
JournoList is not just an isolated example. There was at least GameJournoPros (no good source link unfortunately), explicitly modeled after JournoList and it's safe to assume that many more exist.
[0] :: https://en.wikipedia.org/wiki/JournoList
[1] :: https://en.wikipedia.org/wiki/JournoList#Cabalist_spin-off
I see this argument echoed a lot and I doubt it's as overt as this. Maybe journalists inadvertently have some bias towards social media platforms because they see it hurting their industry or leading to a fight over clicks rather than quality content (which I do believe most journalists would prefer to create rather than clickbait). I find it harder to believe that journalists have banded together to fight all of the platforms that are potential competitors to them; an individual journalist probably just doesn't have that incentive and a news org probably couldn't keep a coordinated activity like trying to fight social media companies secret (I mean, trying to keep a secret cabal among journalists must be pretty tough)