That Google Support post was updated awhile back. You can avoid Google's automatic rel="nofollow"
on external links IFF you have your own non-Google hosted domain, or maybe any URL, pointing to your Google Site according to this follow-up:
Google just updated Google Sites so if you have a mapped web address
pointing to your site, [e.g. http://www.example.com], then the
rel="nofollow"
will not be included on any links on your site when
viewed at the mapped address, but will be included on links at the
sites.google.com address.
Other than that, the same nofollow remains in place for Google Sites. Google hasn't responded to any of the inquiries requesting it be changed.