Is this false advertising?

[Cala]

It depends on what the small print said. It is more than likely that the small print has a clause stating that they reserve the right to change the offer at any time. This is common practice and I would be very surprised if this clause is not in there somewhere.
So, you need to check the small print. If you can't find that clause, or something similar, then write to them and demand that they send the incentive that they offered.