I think you're arguing tangentially to the point being made, which is that: no insubordination happened. They were simply critical of how Musk represented them.

I have to assume they knew when they penned the letter that they would find out whether their leader could take criticism and help them make a better company and product together, or react immaturely and let them know that their time would be better spent elsewhere. Seems they got their answer.

In any case, yeah, Musk owns the company and has the right to fire people for criticizing his business decisions. Bold strategy, we'll see how it turns out for him.