They never explicitly stated that they curated the junk, but I’d vote that it’s unreasonable to expect them to. It was a game show, after all. It’s like expecting them to clearly make it clear that the host of Jeopardy doesn’t actually know the answers, other people have written it all down on cards for him; making sure that we know that everything they have the contestants eat is completely safe and nutritious to eat; or explicitly stating that the primary criteria for participant selection was how high they ranked on the eye candy scale. These are all things that, yes, maybe goes against the spirit of the show in some way, but they make sense for ratings, and it’s not like they’re really trying to keep it a secret.
I’ll also note that often you would see things like 5-6 engines in the yard - more than the contestants needed - so where they could get junk in bulk that had the sorts of things, they went for it. They didn’t just pre-buy exactly what the contestants needed. It was just in the case where specific, expensive things were required - like rocket engines - that they would buy exactly three and put them in the yard.