I've been doing a bit more digging around, mostly as a way to avoid doing real work.
Found this from Jules Urbach, CEO of OTOY:
This is interesting. It was exactly a month before that comment from Beppe, the Octane developer.
He says "it works", which seems to imply "we (or someone) have/has tested it", and he gives a specific version number, implying a change was made to provide that support. Which would in turn imply they had drivers to test that against.
Octane X is a macOS-only product, so there's no question of him talking about other platforms, and it was macOS App Store "app of the day" recently, so it's definitely a product that Apple supports. Further evidence of OTOY's involvement with Apple is a news article by FXGuide which says
"Apple has been working with OTOY to bring the Octane X to Professional Mac users since last year’s WWDC." All this gives credence to the idea that OTOY may well be getting info, and even code, that the rest of us are not privy to.
Beppe said he had no 6000-series card to test on, though also mentioned that that's because he's in Italy; Otoy are based in California and I guess Beppe works remotely. Still, the CEO could just mean that they've ensured Octane X will use the cards if they're found, without actually trying it. But even if so, could that be done 'blind', without any Metal driver? He didn't say "it should work", he said
it works, and gave a specific Octane X version. Admittedly the fact that he didn't realise it definitely couldn't work in 11.2 does also throw a little doubt on his deep technical knowledge on the subject.
Anyway, more speculation, but this could perhaps be support for the idea that drivers do exist and have been tested against by some devs outside Apple. Combining this with Beppe's comments a month later we could surmise that Otoy were given early drivers to test against and were told they'd be released to everyone in 11.3, but then that was delayed without anyone informing Otoy (or at least, Beppe.)
(I wish Apple would get the hell on with finishing 11.3 already so we can see 11.4b1!)