This seems to only be relevant to people already at risk of PSTD (troops). I think it says, having a concussion makes these troops more likely to develop PTSD.

This seems to only be relevant to people already at risk of PSTD (troops). I think it says, having a concussion makes these troops more likely to develop PTSD. But troops are already likely to develop PTSD! This biases the control group. You're confusing 'increases the likelihood of' with 'causes'. If you could demonstrate that peple who suffer a concussion then develop PTSD when non-concussed members of their group do not develop PTSD, then I'd accept the claim that concussion causes PTSD.