It is not permitted to rely on GPS as a primary means of navigation. Nothing, I suspect, to do with suspicion of GPS accuracy in this case - my guess (and it is only a guess) is that this A320 was fitted with INS.
Similarly, INS is not to be used as a primary means of navigation.
The question, however, has to be asked - in this situation, which would have been more accurate - GPS or VOR/DME?