I've never heard of this. Why do you think this is true? Is that because QM is our first example of inherent randomness in physics and all previous physics was deterministic? I don't understand the argument you are pointing out
No, it's a purely logical argument. Randomness is fundamentally unprovable/unfalsifiable. Therefore, in principle, any possible process in any imaginable system can only ultimately be found to be either provably deterministic or unfalsifiable.
On the other hand, it is possible for there to be unfalsifiable structures which determine the outcome of apparently random processes which are not truly random. In fact, it's trivial to create practical cases of such sources of psudorandomness.
You believe that positing the existence of one such structure to be the bigger leap simply because it may be not just practically unfalsifiable but in-principle unfalsifiable (though that is undetermined).
This is the core of your disagreement. You suppose Occam's razor dictates something appearing unfalsifiably random is in-fact random. The other side supposes that you're the one making a stretch, since everything which can be falsified categorically turns out to be deterministic.