I don't think it's credible at all, just another attempt to make the leak seem Russian: the "Russian Files." It sounds ominous, but the technical details don't make sense. It's like someone with Stieg Larsson novels as their sole technical background made this up...

Mysterious hackers have to move these files across the "Deep Web" like physical contraband, but unwittingly leave behind shadow images that are somehow still discoverable "at specific locations."

They use the term "Deep Web," usually just the unindexed areas of the internet, but they seem to mean a darknet like onion sites or freenet. Yet they don't mention that sort of thing because they have no specific knowledge of how that stuff works.

It all sounds like someone who has read or heard these technical terms before has come up with a scenario to implicate mysterious Russian hackers. Without understanding the technology, they come up with something that might sound good in an action movie script, but really makes no sense at all:

When Clinton associates thought they were deleting her files, they may not have realized that files leave images even in the Deep Web when moved by hackers.

I'd expect something like this to emanate from Clinton's campaign, who have weaponized the Dunning-Kruger effect in 2016.