Solaris broke off from BSD and is based on System V Release 4 ... which is partly based on BSD. It depends on how far back you want to prune the tree. They just wanted to stick with the pure BSDs that didn't take come from other projects I guess
AFAIK, Solaris doesn't share any code with the 'UNIX' this talks about, or at least historically didn't (dtrace and ZFS got ported to at least some BSDs; some other code may have moved later). At the very least, adding Solaris would make this a forest, not a tree.