They don't want to budge. They look like allen bolts in the picture, but look smooth in person. I suspect they are press-fit. The 2 pieces of protruding plastic (looks like a : in the picture) are definitely there to stay.
In fact looking at this picture (from the fen-tek industries site)
It's pretty clear that it's not just my unit.
It's a shame. The mounting system is simple, robust, allows infinite control over splay, and
I give up, perhaps the keyboard meant to slide off the stand...