I think the basic flaw is expecting a virus to work in with Bluetooth. Whether or not it's "Delta", all any virus needs is a bit of luck and it's on to the next recipient. This is, and remains, the basic flaw with the app. Trying to use technology, not designed for the purpose and built across a variety of platforms, to determine whether someone is at risk of picking up a virus was only ever going to have limited success. Good stick, poor crutch.
To make it useful, you'd really have to have the phones recording every phone they came near, regardless of time or proximity. 100 people in a shop, that's 10,000 interactions. Following Murphy's law, it might of course be 9801 as one person didn't have their phone on and unfortunately, that one person was the Covid carrier!
On the other hand, log everyone into the shop, 100 records and the possibility of 100% compliance. I dislike the Covid check in system but from a practical point of view, it's far more likely to be effective.