Discretion is an elastic concept.
It would appear that it was shown in this instance at Exeter Central, but not to its full extent. A Penalty Fare could have been charged.
TOCs▸ have queueing time standards written into their Customer/Passenger Charters, but they are not backed up by any legislation in favour of the passenger. The best you can hope for after being charged an Anytime fare or Penalty Fare when an Off Peak fare was available is to take a complaint (after paying) to the TOC's Customer Service department. You may though have to prove, or make a reasoned argument, that your queueing time exceeded, or would have exceeded, the Customer/Passenger Charter standards.
FGW▸ 's
Passenger Charter says:
You should not have to queue to buy a ticket for more than five minutes at peak times and no more than three minutes at other times. Peak time is generally considered to be between 0700 - 0900 and 1600 - 1900 Monday to Friday. Regular checks will be carried out to ensure that these standards are consistently met and any shortfall rectified.