From your own source:
“For the 20mph sites (which are not thought to be representative of all 20mph roads), the average speeds were above the speed limit for all vehicle types, ranging from 22mph to 28mph but below the average speeds seen on the 30mph roads.”
So the average speed does decrease, increasing safety. Just because the effect isn’t a perfect 10 mph reduction doesn’t mean that it does nothing.
This means the proposal is effective, but it could be improved with traffic calming measures.
Small tip: don’t say “genuine asexuals” as if the ones that do masturbate aren’t really asexuals. All asexuals are just as genuine and valid, don’t gatekeep.