It sounds like there are feelings of collusion among the owners, concerning players threatening not to play under a tag.
I'm not sure if anything can be proven. It is quite possible that the rest of the league looks at the Browns and is declaring them idiots for the contract Watson got. It is also possible that the other owners have indeed had off the books, face to face conversations and agreed "never again." IE, actual collusion.
I trust the owners to be at least a little sneaky and would not be at all surprised if they did collude--you don't become a billionaire by being nice. How to prove that, even if it did happen, I have no idea. Presumably they wouldn't be dumb enough to write it down, but know knows?
I do like the crazy conjecture that Snyder, purportedly trying to sell the Commanders for ~6 billion, could trade for Jackson, offer him the guaranteed contract, set the precedent for real by giving a second quarterback a fully guaranteed contract, sell the team, and walk away with both middle fingers raised at the NFL.
The only hiccup would be putting the contract money in escrow, but I'd guess that the Commander's new owner would have to square up in some fashion.