I find that very hard to believe. Of course it’s all premised on having the specs. Whether they’d be able to reverse engineer it is a whole different magnitude of problem. It’s true that back in the day, high speed channels tended to have a high degree of parallelism, whereas USB achieves remarkable speeds through high-speed signaling over just a few conductors. That would have been tricky and certainly not have been a cost-effective technology in the 60s, but I’m not sure it would have been impossible to build if cost was no object. Such an adapter – perhaps supporting hundreds of serial lines over a single USB 2.0 connection – would have been the size of a refrigerator and probably cost hundreds of thousands of dollars, but I suspect it probably could have been built! The main reason USB is relatively recent is that it had to be cost-effective for the intended market.
The basic serial line communications controller for the PDP-10 timesharing system supported 64 serial lines at up to 100K baud each for a total throughput at the back end of 6.4 Mbits/sec, which is about half the maximum theoretical speed of USB 1.1. That’s the kind of thing I was thinking of. No doubt the back end used thick parallel cables but only because that would have been the easiest, most cost-effective way to build it.