Fascinating but is that actually true? As far as I have been able to search their implementation seems to have been called pdtar.
I'm not saying that I disbelieve you, I just haven't been able to find any references for your claim.
ChangeLog.1 and NEWS only go back to after it was migrated to the GNU project (http://git.savannah.gnu.org/gitweb/?p=tar.git;a=tree), his personal site, http://www.toad.com/gnu/ seems to be down but an archive (https://web.archive.org/web/20180623140434/http://www.toad.c...) just says 'In 1985 I wrote the "pdtar" program, which eventually became GNU Tar.'
He clearly uses the name "gnu", but did that usage predate the GNU project? I haven't found a reference for that.