I think this is an exaggeration. Witnesses often try to be as helpful as possible and in doing so may supply too much information they themselves aren't able to verify. Of course that can prove to undermine their statements in the first place, but I don't believe attesting memory failures addresses the core of the problem with witness reports.
And yet, direct witness statements from two helpful people standing next to each other, made immediately after the event can vary so wildly that you would wonder if they are describing the same event.
paraphrased from "The Invisible Gorilla: and other ways our intuition deceives us" (Chabris and Simons)
