Contracts are funny things. They are expected under the law to be fair for both sides.
It's entirely likely that a contract will forbid an employee from saying, publicly or even privately, "I think my employer sucks!" It would be quite unlawful for that contractual term to apply to the entire life history of the employee. In other words, the company couldn't fire the employee for something said while the contract was not in effect.
It is also possible that the company could include language in the contract that says the company can release the employee if the employee fails to reveal some part of their life - at any time - that may harm the company, such as a criminal history or even public comments about "I think this company sucks!" But in this case, the company always knew about the events under question, and it would be quite unlawful for them to fire him about them now.
Which is why, in the end, Disney spent weeks negotiating with Gunn for a financial settlement to get out of the contract that included using Gunn's script if they do decide to film the next movie, rather than just giving him the finger and booting him out the door. Contractually, they were unable to do that, but Alan Horn backed Disney into a corner where that sort of costly negotiation was the only way forward.