Although it is stated as a requirement, on the couple of occasions I've used Diamond Buses, the drivers had no desire to see an Oyster card before offering the Oyster Match fare and just charged it as standard to all passengers travelling within the relevant area.
With the overwhelming move by London Bus passengers away from Oyster towards contactless, I suspect they get more money overall this way - after all the sole aim of the Oyster Match fare was to encourage passengers that would otherwise wait for a (cheaper) London Bus not to do so.
I am, however, mildly surprised that the Oyster Match fare wasn't pulled when they joined the £2 fare cap scheme. I would've thought the extra subsidy (based on the "full" fare) would have massively outweighed the few passengers they'd lose by being 25p more expensive than London Buses.