If you'd bother to actually read the article, you'd see that it's precisely arbitrage that is a major factor why a system consisting of unbanked ATMs would be a superior pricing mechanism.
Yeah, seems I scrolled past that section, sorry. "TL;DR"
Ideally there should be a single big market for each item. Partitioning the market into separate exchanges makes arbitrage possible and profitable; and the profits of the arbitrager (like those of the insider who does front-running) obviously come out of the pockets of other traders. So, the more partitioned a market is, the less efficient it becomes.
Anyway, IF the cash compartments have to be kept separate, for the reason I pointed out, then the UBA is like two independent ATMs, one that only dispenses cash for nitcoins and another than only sells bitcoins. Then there is no point in changing the price in one of them depending on the state of the other.
Raising the price in the first one as its cash reserve gets lower may be an interesting idea, but why don't cofee machines use it? Perhaps because the capacity of the cash compartment is an arbitrary choice of the ATM company, so the price rise would be seen as arbitrary and unfair? Because a competitor could install next to it an ATM that sold at a fixed price?