To me it rather depends on whether they put you on the flight 12 hours later, in F. In that case I think I'd regard it as a reasonable, if not brilliant, offer. If you end up travelling anything other than F on the next flight then obviously further mor substantial compensation would be warranted.
Did they offer you a downgrade to J on the earlier flight? I'm gueesing not and that in fact they were over-booked in J so op-upped someone to the one working seat. Otherwise it would seem very strange to IDB 2 people for one broken seat.