Weak. McCabe was fired for lying to the IG - that’s cause. Sending text messages with political opinions isn’t. At least not normally.
You never know, I’m interested and so should you be. If the fbi terminated him for an actual cause, such as acting with bias in an investigation that should be neutral that is substantial. On the flip side, if they fired him for expressing political views on a government issued phone then that is not consistent with past practices and would suggest his termination was politically motivated.