Lots of people are offering suggestions on how to evacuate a station. As
none of us know the real story in this situation it is pointless to continue
to comment.
Even if the staff were useless, rude and incompetent the station still
needed to be evacuated.

It is normal procedure to put people on a train if one is available. The
main aim is to clear the station as quickly as possible. To evacuate by
train a member of staff would advise the driver of the situation and then
both would manually open the doors to let people on. The member of station
staff would normally gather everyone together on the platform so they would
be put into one carriage.
As this can take time asking people to leave may be the quicker option. I
think you would find at stations like Highgate staff are more likely to
evacuate by train because of the lay out of the station.

One thing I do know is that everytime I evacuate a station I learn something
new. So lets hope the staff at Farringdon will do better next time.