Rail has a channel for 6mm/7mm bead and awning comes with a 6mm bead attached so they seem compatible and I don’t think you’ve overlooked anything (assuming you don’t have a pop-top as that rail not compatible with a pop-top).As far as I can see both these items are compatible
VW T5 & T6 SUN CANOPY & DRIVEAWAY AWNING BOLT ON AWNING RAIL ONE PIECE & HIDDEN | eBay
REIMO PALM BEACH 2 SWB 2.6m Sun Canopy Dome Shaped Awning For VW T4/T5/T6 4043729140639 | eBay
It seems the most cost effective way of getting an awning. Two questions 1)are they compatible 2) have I overlooked anything else ?