Wow so I read your link, and I don't see anywhere that it explicitly says it was caused by the massage gun, just that it occurred after use of one, and that it may have be the cause or at least contributed to it. Strenuous exercise seems to be a common cause, and since the reported case was someone training with a coach, strenuous exercise doesn't seem that far fetched. But you are correct that the OP article explicitly claims the massage gun caused the rhabdomyolysis.