What an astonishing turn of events!

If they had been boarded on their original service, and then offloaded due to a discrepancy (which with all the ticketing going on I can see could easily happen) then surely that is the point at which BA need to rebook them gratis? They would have been accepted for departure according to contract, and now the contract owner finds an internal problem which the customer cannot possibly fix.

I would send in a short webform to Customer Relations, then call them to see if they can help or remediate. Broadly speaking I would see traffic congestion as not in scope for "beyond your control" (especially in NL, there have been a number super-jams recently, e.g. during the tractor protests). If there was a serious accident on the slip road to RTM that would perhaps be another matter.

As general advice for anyone else reading this, if you are in this situation, ask the Contact Centre to make a note on the booking, but only going into a rebooking dialogue if they were not on (presumably) a Basic fare, which appears to be the situation here, otherwise Free Change on the Day would apply. Then at the airport, see what can be done.

If you were not on a Basic fare then other factors come into play.