it was quite a widely held belief that people like Williams couldn't possibly write all that music himself.
That is no doubt true, but not just confined to those times.
I think that the role of "orchestrators" in film music often generates confusion, especially outside the anointed in the business. JW addresses orchestrators somewhat at 6'44" in the second interview when he says that he provides the orchestrators with a 10-line sketch. Consequently, it makes sense that he characterizes orchestrators, for him (at about 9'15" in the second video) as more "stenographers" than arrangers or co-composers.
Not Everyone Is JW
That said, not all composers have the time to write out everything. Sometimes, hideous deadlines absolutely require the composer to farm out to "orchestrators" significant parts of the composition process, sometimes just "continue like this" with a cue that's half done, sometimes much more, including tempo, harmonic choices, percussion -- sometimes the whole thing. So the presence of orchestrators and confusion about what exactly they are doing gives rise to misinterpretations.
Nowadays, although many composers use orchestrators, in plenty of cases virtually every note is already "in there," whether sketched or electronically, and the orchestrators' job is more like the stenographer characterization that JW uses.
Favourite quotation (interview 2): "You don't have to be good, you have to be strong."
