Heidigger, what does the manual's author base that on? Just because they wrote it doesn't mean a thing. I can write a manual that says that cables and cleaners mean nothing. Would my manual have the same weight as your manual? If so, how would you reconcile the contradiction? Also, appeal to authority is one of the classic logical fallacies.
If you want to appeal to testimonials, then I assert that that cables mean nothing. I've listened to several and they're all the same. I've also had quite a few audio devices from the 1940s through the 1960s. I've never noticed a difference with the old contacts/connectors. Even a good bit of corrosion doesn't matter.
Sure, that conflicts with popular cable mythology. But I've never seen any evidence to the contrary. If you have some, please post it.