Understanding an RF Level detector circuit
Agreed -- as drawn, that wiper connection doesn't make much sense. (I assume you are referring to VR1, not VR2.) The worst part of the wiper connection is that it will tend to short out the RF signal if set to the left extreme, but VR1 doesn't appear to be configured as an RF attenuator.

Perhaps the schematic was supposed to show the wiper connected to the anode of D3 (not cathode). With an anode connection, VR1 would allow trimming of the offset (or effective threshold) of the first op amp through the DC path of R7 and R20.

When RF is present at the shared node of D2 and D3, the diodes will pump current through R7, which creates a DC voltage that the op amps can detect.