I can't help but feel, since many of the same arguments against neovim could be said of the rewrite from vi to vim, that this is the natural order of things and the point of open source.
Sources are at http://svnweb.freebsd.org/csrg/usr.bin/ex/
Keith Bostic reimplemented it from scratch as nvi (new vi) for 4BSD.
Vim's code is not descended from either of those.
¹ http://minnie.tuhs.org/cgi-bin/utree.pl?file=1BSD/ex-1.1/REA...
After all Linux didn't use a SCM until quite late.
The BSDs have been using CVS for a long time so it's plausible they used something like RCS or SCCS before CVS was available.
FreeBSD repo goes back to 1993: https://svnweb.freebsd.org/base?view=revision&revision=2