I think that's pretty much the conclusion of the fitrv guy's tests... and at the time I was looking at this stuff my intuition was telling me the same thing. These things will never deliver the kind of flow rate and velocity to really do a thorough blasting that you are expecting
The way I look at these things, they are merely a way to add water to the tank.... which alone is a good thing sometimes..... although I have read that many experienced folks never do any sort of rinse or flushing of their tanks. in my case though, my usage over weekend trips and such is so intermittent, and with kids it's hard to know how much paper gets put in there, so on occasion I do fill the tanks back up with water after dumping at home, just to get a flush out.... and a couple times that I've tried the clear adapter fitting, I'm surprised at how much more paper and stuff is still in there....
But in the end, do I really care if it just stays in there until the next normal dump.... no really.