It appears that there is a degree of confusion as to whether it was the RAT probe heater or the RAT probe itself that was the cause of the initial return to the gate.
However, whichever it was, there's provision in the (FAA) MMEL for either to be inoperative. For the heater, the provision is:
May be inoperative provided:
a) Flight is not made in known or forecast icing conditions, and
b) ER operations are not conducted.
or
May be inoperative provided an independent RAT system is operative
For the probe itself:
RAT/Thrust Rating System
The RAT portion may be inoperative provided:
a) A SAT or Standby RAT indicating System or PMS SAT readout is available,
b) Other Systems affected by the RAT Probe (DFGS, CADC, Thrust Rating, FMS, OMEG, PMS) are considered,
c) Thrust Rating System portion is considered inoperative for the Series 30, 40, and 50 and
d) Procedures are established to verify engine power setting.
(Taken from rev 37 of the MMEL, available
here)
Either way, there's a procedure for either case, which if followed should mean there's no impact on flight safety. Given the speed of the turn around I'd
guess it was the heater, not the probe itself, which means the scope for subsequent issues is minimal.