Ryanair's friend EC Regulation 261/2004 it states that if a flight travelling to or from any EU country is over 2 hours delayed Meals and refreshments in relation to waiting
time. Two free telephone calls, emails, telexes or faxes.
Source
If you were to be sceptical you could say that this was done purpously in order to attempt to evade paying out the cost for refreshments.
But was the Captain forced to make the decision on holding all passengers on board based on fear of backlash from the management? Had he previously made a request for late boarding but been told strictyl to board on time?
Did the flight pick up the slot before or after they had started to board the passengers?
The one thing that is definite that if they were going to hold the passengers on board for that length of time then they should have been provided refreshments. Unless there is a loophole which Ryanair has found which is the reason they boarded on time.
Yes slots can come forward but if it was due to ATC strike then there is little chance it was going to jump forward 5 hours in an instant.