The "6-hour version of Dune" is probably confusing the Lynch treatment with Jodorowsky's attempt at a Dune film -- which was never filmed (the script wasn't even finished!), but whose production was documented in the 2013 Jodorowsky's Dune:
Possibly, but the rumours of a 6-hour and even longer cuts go back years. I remember discussing this way back in the late 1990s. It's a very persistent rumour.
https://en.wikipedia.org/wiki/Jodorowsky's_Dune
It may also be confusing Lynch's film with the 2000 Sci-Fi miniseries Frank Herbert's Dune, which totalled roughly 5 hours across three episodes.