Ben wasn't "fired" — he voluntarily resigned from the Node and libuv projects because he was so put off by the treatment he was receiving.
The reason they mentioned firing was because they were responding to Joyent's suggestion that he should be fired. It seemed to me that the overall thrust of StrongLoop's post was, "We think Joyent are quite hasty if they would fire a great guy and great contributor like Ben over a misunderstanding."
Hopefully you can understand how even if Joyent was not his direct employer, their publicly stating that he "should be fired" was the main impetus leading up to his departure.
This is why I said that legally he was not fired. You can't be fired by some random collaborator or Joe off the street. It's your employer's job to pay your salary, and to fire you if that's what's needed too. That is not what I'm talking about.
Consider that you can be "fired" from an open source project where you don't receive a salary and like your coworkers, you are providing labor on a volunteer basis. That's not what happened here, but if you take the StrongLoop and the salary out of the picture, it's exactly what happened. Joyent (and the rest of us who are not part owners of StrongLoop) are not getting his labor on an unpaid basis anymore.