It probably wouldn't show as dis. (disconnected) when a modem/router is actually connected to the line.... This homehub 5 has the gas discharge and the DC block capacitor inbuilt, similar to an NTE5
From cursory research, modem’s front end line interface seems to sometimes (/often/always?) comprise a tranformer, coupled by fairly low value caps, 0.1uF in circuits I have found. The 470k resistor does not normally seem to be present.
Assuming that the line test uses a carefully chosen frequency, and looks for an impedance predicted by the correct cap/resistor combination, the modem connection would look quite different compared with the master socket’s R/C.
Not wanting to worry unduly but I’d imagine worst case scenario would be that BT, without prompting, discovered such a “line fault”, and invested significant engineering resources in tracing it. If the ‘fault’ were subsequently attributed to unauthorised tampering within NTE, I suspect they might quite reasonably want to recover full costs of investigation.