I forgot why I couldn't use xargs -P there. If I had to guess ancient DoD version of RHEL, like say RHEL 5. Note it was released in 2007 and xargs as you mentioned had that since 2009.

Sometimes Redhat backport things but I just remember not finding anything there and then being very happy to have discovered make. I think even with xargs -P I would have gone with make anyway as it involved a few dependencies and checking of file times.