This may be true, but it's just a guess. We don't have the same amount of data to show longterm effects of MMA. Dementia Pugilistica is real, but we don't know yet if it soley is limited to pugilists.
The repeated trama due to the protection of headgear and padded gloves is what causes brain injury.
I would never let my two sons box. I called for the removal of headgear from amatuer boxing all over Maxboxing.com and Boxing Scene long before it was done. Probably 15 years prior.
However, i would let them play football as i believe that CTE is likely caused by guys who abuse steriods/drugs and suffer concussions, and not just by concussions alone. Maybe in 15 years, Will Smith will do a movie on it. ;)