Buried deep in plaster and brick work the worse that would have happened is the cable overheated in a safe contained environment and either cut its own throat or shorted out which is when it would have been looked into.
Recommended IR readings are their to indicated a breakdown of the insulation barriers between conductors as well as down to earth... if the readings are well within permitted values it is usually an indication that the public are not at risk of shock due to a breakdown of insulation barrier (the dry building material served as a good barrier in your case) although its clear occasions exist when this isn't true as without a conductive contaminant 2 bare wire mm apart with give good IR.
If the earth was broken prior to it been pulled out then this would have been found and identified with a correct sequence of tests - assuming the cooker was fully disconnected during the R1 + R2 tests then a parallel earth path through the gas pipe wouldn't give a false positive.
We have to Note here badge that the full sequence of tests are our best way to find faulty or dangerous circuits but can't always identify every possible combinations of situations, their are many tests we can't do and as matter of assumption we just carry on in the blind believe that that MCB will operate at the correct time-curve when overloaded or will trip instantaneously when the S/C value is high enough... we only really check rcd's for their earth leakage reaction and functionality so here we have another void of assumption that our test sequences don't confirm.
At least with rewirable fuses or cartridge fuses as long as the wire fitted was made to spec it would always operate at a predictable and measured value and also left traits of the cause of the wire fusing.