The offer says the car has to be delivered by March 2021… Is that likely to happen with current wait times???
It would be upto the customer and dealer to agree a remedy (preferably in writing at the time of order) if for example, the vehicle does not arrive in time to meet the requirements of any offer.
One thing for certain is that Motability will not get involved as they do not condone dealers offering inducements or discounts/cashback etc off the published Motability advance payments. They simply will tell you any such offer/inducement/cashback etc is a matter between yourself and the dealer.
As with all such like offers a degree of ‘caveat emptor’ applies, hence the need to get such matters in writing at the time of order.