Neuroimmune activation in TBS is a rather well-established fact. One could question whether that is the cause of post-concussion fatigue. But neuroimmune activation is also a rather well established as a cause of fatigue, such as age-related fatigue. (https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2017.00208/full)Many people with post-concussion syndrome suffer from something akin to PEM and I don't think anybody has any idea on what the cause of post-concussion syndrome is.
I don't know what is something akin to PEM. But I don't remember PEM being mentioned in any personal account for post-concussion fatigue I've read. if PEM is a part of post-concussion fatigue, it apparently is not as prominent as in ME/CFS.