technically, a "forward auction" is one in which a single item's price goes up with bids, a "reverse auction" is one where the price to provide the item (usually a bid for work, or procurement) is driven downward by extra bids.

You are correct that a technical sense, it is usually said that any auction would require multiple bidders, and if we want to be even more technical, nothing about what UA is doing is what we think of as an English Auction in that it is not open with others knowing the bids.

I have used the term to describe UA lowering the price it offers the upsell at over time, in essence bidding against itself. I think its probably the closest common term to what UA is doing. If you know of a better closer term, would love to know it.