As far as I know, all of the BSDs have adopted code from each other over time. It might simply be a side of effect of the common heritage and license, but it seems to be one that none of the people involved have a problem with and often consider desirable.

But that does not make Dragonfly a "research platform" for FreeBSD. Both systems have different priorities and goals.

(Although I wonder if any of the things that Dragonfly set out to do differently has been adopted back into FreeBSD?)