I don't have METAR/TAFs for the destination and other airports close by, but based on the METARs for PMI and that possibly a local weather pattern known as DANA, Depresión Aislada en Niveles Alto, was affecting the weather in the whole area, so it sounds like bad weather would have been forecast. Haven't flown in that area for a few decades, but used to fly for Air Nostrum (via ACMI). I now fly the A320, not the 737, and in freedom units, so forgive me for any mistakes.
It sounds like they were planned to fly for 2:30, and ended up flying 3:26 after holding at FL350. Guess that holding would have burned around 2000kg, and they landed with 1120kg (questionably precise amount). So the plan was to land with around 3000kg. Alternate fuel would have to be at least around 800kg for either Ibiza or Mahon, and minimum was 1200kg. So they left the UK with about 1000kg extra.
Do these numbers sound about right?
If so they departed with less than 30 minutes extra fuel into questionable weather, and held over BCN at FL350 for over 30 minutes ( based on total flight time) and still decided to continue to PMI, knowing they would get there with adverse weather and no extra and less than alternate fuel.
Even if you agree with the concept of committing, that does not apply here. It would be different if they were holding a low altitude over an isolated airport, where the divert option would be unsure. They were holding at high altitude, and were right above an airport (with even more runways, not that that matters if there is weather.......), so definitely they had other options.
Typing with a drink in hand, in a comfy chair, firmly on the ground.