Employers have no intrinsic right or expectation of confidentiality from employees in regards employment status or for the general reasons they quit. In other words, posting "I no longer work at Company X because I do not agree with the company's policies" is not a breach of confidentiality. Even if the company had severance non-disparagement clauses, which the NLRB has been frowning on for rank and file employees in recent rulings, the wording of the ex-exmployee public statements made so far is so vague they would likely not apply. If a former employee divulges trade secrets, that's a different discussion, but no posts I've seen have done so.
On the other hand, the employer made a public post implying a stance for the former staff on the AI/tech issue. At best, the implied position is unclear. At worst, he implies the staff quit because they oppose the policy he posted--that is, they suppoprt the use of AI and blockchain in RPG product creation. If so, that pro-AI use position would have negative career implications for those ex-employees, given the general animosity toward AI among creators in the RPG market. He then refers to what are likely confidential employee communications to shore up his original implications, even while denying the former employees the ability to do the same.
Might those resignation letters contain trade secrets? Possibly. So he should not have selectively cited them publcly in his bid to show "transparency," particularly in defending the original post implying the stance of staff on AI issues. To defend themselves, they would have to refer to their communications with him, even just as summary, which could get them in legal trouble.
He's the one with the power here. The portions of the public posts where he even suggests employee positions is a bad use of that power and, inasmuch as they breach employee conmfidentiality, are not a proportional response to the original employee posts.