Does it mean that the new wikis will have some
content which is
GFDL-only (what was created or derives from something created elsewhere)
and some content which is doubly licensed?
As I understand it, yes. Which seems extremely confusing, and likely
to cause more problems than it solves. The only explanation for this
very strange decision that I can see if that the new license has an
arbitrary date in it that pre-dates the actual publication of the
license. If that's the case, then the correct action for the WMF is to
point out to the people writing the license how stupid this is, rather
than trying to cater to such a stupid clause.