Points taken.
At the same time, your link shows that of the 178 aircraft involved in collisions, 96 were gliders and 68 powered GA aircraft (excluding glider tugs). Powered aircraft collisions are more likely to kill someone (0.62 versus 0.26 fatalities per aircraft). Collisions between gliders and powered aircraft are rare.
The UK GA register shows 1750 gliders of about 13000 aircraft registered to fly. So all in all it's clear that gliders are much more likely to be involved in collisions than powered aircraft and despite the lower lethality of such incidents, they do account for the majority of mid-air deaths.
So I think it makes sense to treat gliders and powered aircraft separately.