could* be at a lower potential
than the cause of the fault... and the fault current will flow through
the bonding rather than the CPC

Assuming that no one has at some time put plastic push fit connectors into the
plumbing system . This would cause a fault current to find another way to earth
other than via the bonded metalwork and or possibly the cpc. There may not be
a requirment to test for continuity but it makes sense to when messing with
power showers and water that was my point really.