I can confirm that the several sources I read online were correct. There is fluff supporting that Ork tech works due to a psychic field. You can find it on the last page of the 3rd Ed codex, in the second to last paragraph of genetor Lukas Anzion's report. I'm not going to bother with quoting it and I don't really care if it's how Ork tech works or not, but the fluff is there and you can read it yourself.
I'll quote it:
However, as disturbing as it sounds, these 'facts' become true. Red Ork vehicles do travel perceptibly faster than those of other colours, even when all other design aspects are nominally the same. Similarly, many captured Ork weapons and items of equipment should not work, and indeed do not work unless wielded by an Ork. I believe this is linked to the strong psychic aura surrounding all Orkoids and have developed the Anzion Theorem of Orkoid Mechamorphic Resonant Kinetics. I theorise that many Ork inventions work because the Orks themselves think that they should work. The strong telekinetic abilities of the Orks' subconscious somehow ensure that the machinery or weapon functions as desired."
This is a great example of what you earlier called "two sides to the argument about a piece of background". This is an in-universe, Imperial explanation of Orks. It opens up interpretation of not just Orks, but the Imperium's understanding of Orks. Can we say with certainty that the weapons captured are actually mechanically unsound? Or is it that it ecplises the writer's understanding of mechanical technology, and he can't fathom to imagine that Orks would be more advanced? That second bit is doubtful, as in a prior piece of fluff he mentions that Orks are just as intelligent as humans, and in the same piece of fluff as the resonance, Anzion talks about meks having "...an encoded knowledge of basic physics and mechanical engineering theory." Alternatively, maybe the resonant field he mentions actually "fills" in the gaps of individual weapons when they don't work right, since an Ork wouldn't take the time to inspect his weapon to check that it is proper. Finally, maybe it is just Ork magic that makes everything work. This is a good piece of fluff, because it leaves enough mysteries that can be argued over, and inspiration given to the readers.
But this tangent of a tangent didn't start cause we were debating if Ork resonance made their guns work. It came about because I mentioned that Orks of the past, especially before the War of the Beast, wouldn't be the same as the Orks of 40k. You insisted Ork fluff was inconsistent, to which I disagree. The bit I took issue with was:
One example given was an Ork tank that ran because it had a drawing of an engine in it.
Which, unless I am very much mistaken, simply doesn't exist. If it does, then that is a gross inconsistency, and I would pray that no one at GW
thinks to build on it in the future. But, so long as it isn't a real piece of fluff, I stand by my point that people take Ork resonance out of context and apply it to just plain silly things. It is a powerful effect, but it isn't the end-all, be-all of Orks. And their fluff has been remarkably consistent in this regard, going back the 28 years I have available to me in codices and sourcebooks.
So, on topic, if GW
were to make a game set even further back than the Horus Heresy, I don't believe we can just assume Orks could be transplanted as they are now. They certainly wouldn't be smarter, they wouldn't have Gork and Mork around to inspire Gargants and enhance the Ork Resonance. Considering the problem they gave the Emperor and the Great Crusade at Ullanor, a strong Ork empire would probably still be problematic for the Eldar empire and the humans of the Dark Age. But I don't think they'd fit with the setting of that game.