There is nothing to suggest ispell-nl is GPL-licensed. There is no explicit license whatsoever for ispell-nl itself, but the word list it is based on comes with this copyright notice: ftp://ftp.tue.nl/pub/tex/GB95/spell-nl-v5b/COPYRIGHT Clearly, by only mirrorring specific ispell-nl files, Gentoo is not mirrorring anything complete.
Plus, an ebuild should never download a file with such a generic name as README, and even worse, an ebuild should never hardcode ${PORTDIR}.
Yuck; that ebuild is horrible! You're right on all counts, of course. Committer has retired, else he'd have been up for right good spanking... I guess we're going to have to build some tarballs and mirror it ourselves. The only way I can see to comply with that condition is to supply the relevan bits of ispell-dutch96 (skip the w32 bits, fex) and all of spell-nl-v5b even if the latter isn't used by the installation.
The old ebuild is gone and has been replaced with a newer version which contains the official 2005 spelling and is licensed under LGPL-2.1.
