I have been working for the company too recently to be able to definitively answer you if and where the management is making mistakes in management.
Please accept my opinion with the caveat that I am still getting my bearings in the organization and the people within it.
The comment you described by our former employee is extremely extreme. The publication of a personal opinion on a public site with the sole purpose of discrediting the company. The character of the writing speaks of a person who feels humiliated and disgusted. If I could still point to one item to work on, it would be the employee exit interview.
At the end of the day, even if we are not happy with someone and the decision has been made to part ways with someone, that person should receive proper feedback from their line manager about their performance in the workplace. If the employee in question was not given an attestation, after which he would understand his shortcomings or the requirements in his work, then this would lead him in the direction that he does his job conscientiously and his release is incorrect and subjective.
It may be a good idea to speak to the Program Manager or Functional Project Manager who will have a more complete understanding of how our former employee worked. And they were also his direct supervisors, i.e. they are familiar with the nature of his presentation and personal character. Read more: “Project manager vs Program manager vs Project sponsor“, https://eduwiki.me/project-manager-vs-program-manager-vs-project-sponsor-differences-and-responsibilities/
Another unpleasant part of his statement is that he sent a strange email to our client, to say the least. This is where I think management should step in by sending the customer in question a repeat email with a brief explanation of our employee’s behavior. The email, I think, should not sound apologetic, but rather decisive on behalf of an organization that values its customers and for which reputation is a significant element of its values. It is even appropriate to make a personal meeting between you and the head of the client company. After this meeting, I believe you will have more clarity on how we are perceived as a company executing the project and whether we will be successful if we continue with its implementation.
In my opinion, it is important to identify and observe the replacement of the fired project manager. From it, you can get direct information about the team of programmers. How they work, how they reason, how they perceived their previous boss, etc. Of course, the Program Manager could also have a view of the work of the programmers. When you have the full picture in front of you, then you will be able to take an objective opinion on whether it is necessary to release any of them.
Last but not least, I think it is reasonable to prepare and publish a response from our company on the Internet. Objective, with a fair and neutral tone that presents the facts. The public image of the company as an employer is no less significant against the background of strong competition and the lack of qualified personnel in our sector.
I hope, in my opinion, I have helped you by adding light to the problem from a different angle.