The signalling lamp could have been either the masthead all round signalling light, or the hand held ALDIS lamp as someone already stated in this thread.
I use both regularly at sea, and find that a lot of the time they are far more effective than attempting to use the VHF, particularly if a target is not easy to positively identify.
Lights have a far better range than whistle signals in good visibility, and are more likely to be noticed by a lookout who may have other sounds, noises and alarms on their own vessel which may affect their listening watch.
Whilst I'm fully aware "All available means" is an excellent all encompassing phrase used in the collision regulations, it's worth pointing out that whilst signalling lights are specifically mentioned within those rules, the use of radio equipment in collision avoidance is not.
And - before anyone says it, it is not fair to say that the rules have simply not been updated to encompass the use of radio, as annex IV is completely up to date with regard to radio and gmdss installations.
I'm somewhat disappointed that some would assume the master of the container vessel to be lying in his statement regarding the use of a light signal, which is a perfectly reasonable act of good seamanship.
I'm no expert on VDRs, but I have listened to many recordings where the use of the ALDIS lamp is audible to the microphones, as the shutter makes a clicking noise during use.
I'm sure the navy will release the track, but all I good time when their internal investigations are concluded.