That does sound crazy.
If I were to offer you a contract stating
If you give me $x,
I will repay you $ x + interest over a period of y months,
I will default on the repayment, in which case I will give you a z, a thing of unknown value.
you’d be foolish to take it. If you were willing to take it, I’d be foolish not to offer it, default, and give you the lowest value z I have.
The mortgage you describe sounds like this contact.
However, there’s a difference between what television adds promise, and what the agents and lenders to which they are meant to steer people will actually offer. And there’s a difference between what is actually shown in a TV add, and what we may see, especially if not paying close attention. I’d have to actually see the offer you describe to believe it really exists.