Those charts don't look anything like the airline specific charts which I can assure you have a number of approach altitudes on them.
This incident was quite simple really, when cleared for the approach the crew would normally set the vertical intercept point (VIP) altitude which for RWY05 is 6300'. You would then normally engage managed NAV so that the aircraft flies the managed vertical profile in accordance with the chart. At this point it is up to the crew to basically 'monitor' the approach and make sure the constraints are met. It would seem in this case for some reason once 6300' was set that the crew selected Open Descent which means the aircraft will just descend to that altitude regardless of constraints. Although not recommended to use 'selected modes' on RNP approaches it is allowed, just means you need to be extra vigilant about the monitoring of the constraints, I suspect in this case the crew did not intend to be in selected mode. The failing here is it would appear that they then didn't monitor the profile (really the only thing you should be doing!!).
Although on this occasion this error didn't endanger the aircraft, as we all know the act of descending below an instrument approach path can kill you and is a serious incident.