More like there were a shit-ton of 28nm spares lying around Nvidia from all of those unsold Shield devices, and Nvidia was selling them for very cheap.
Which begs to question: What happens after they sell the lot out of those 28nm chips?
This is not how custom chips work.
There is no "pile of spares" laying around for a chip that is custom made. These things are fabbed fresh, and even still there are no Maxwell Gen2 chips on 28nm to be laying around.