While this does not mean higher-ups approved her messages, it does show that she did it by the book.

If the higher ups felt this message was inappropriate and should be removed they could follow the same procedure instead of firing the engineer

You have to satisfy the criteria of labor law, which is (working from memory here) that two or more employees come together in concert around a workplace issue. At that point they are engaged in protected concerted activity and you can't just fire them.

I don't believe the code review meets this test, but I'd love to hear from someone who knows what they're talking about.