I can't think of any reasonable reading of the statement (in which they directly say he's throwing a tantrum and why they think he is) that can be read as an allusion to a scare-quoted "tantrum" about a contract dispute. They don't put it in quotes, just straight up say it and their objections, whatever you think of them, are not about contracts.
I'm not sure how you can accuse me of both scare quoting, and actual quoting the same word at the same time.
And he has listed in his contract that he can publish without editorial oversight. The emails from the editor are very clearly are drawing a hard line that certain themes won't be publishable, in contrast to his contractual rights. Him taking offense to that is the basis for them claiming that he's having a "tantrum".
It's very clearly about contracts.
As far as I can tell, the "tantrum" they allude to is better stated as 'exercising his contractual rights he negotiated as a founder for exactly these sorts of cases'.