Contrary to what happens to the blood inflow, outflow in the supine position, through vertebral and internal jugular veins, is more than twice the outflow in the sitting position (739.80 ± 326.32 versus 278.24 ± 207.94 mL/min).
What they may be saying is that there isn't so big of a change in the inflow as there is in the VVs and IJs when going from upright to prone. That means we are missing a big piece of the outflow, on the order of the same size as them combined. That would be the other collateral veins, measured indirectly, but measured, by Dr. Zamboni.
I would make a table:
-------------------|----Jugulars----------|-----------Vertebrals----------|-----------Other collaterals
upright flow
prone flow