It was a localised power cut. IT systems were also affected, including gateline computers, so it wasn't just lighting which had no (working) backup. As Three Bridges is staffed 24/7 I can only suggest that this incident may have just occurred and it is entirely possible that information had not yet reached everyone it needed to. As there is a regular overnight service it is not like the problem can be left until someone turns up a few hours later.
There is, of course, portable emergency lighting equipment available. It's actually quite good if it works properly. The other issue is with that: I've known the same type of emergency lights simply not to start up on occasions, for what I can really only describe as unknown mysterious reasons (batteries were all charged etc.).
If a train calls at a station with no lighting it is mandatory for it to wait until the platform is clear of passengers. Obviously drivers of DOO trains (with no platform staff to dispatch them) have no realistic way of being sure of this without walking down the platform, so usually they won't call as it takes so long. You also have to factor in whether the stock has porter switches to lock out each coach individually, if needed. If there is little ambient light then sometimes conductors will have to do so as part of their risk mitigation. On some platforms at Three Bridges, this may not be a problem.
If platform staff were present then a clear understanding may have been in the process of being reached with Control and the signaller about whether they could see enough to dispatch DOO trains, or calling the on call manager. Both of those things could be necessary actions which might involve being positioned away from passengers in one of the offices (of which there are several at that station).