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.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).
I would make a table:
-------------------|----Jugulars----------|-----------Vertebrals----------|-----------Other collaterals
upright flow
prone flow