BSD derived directly from AT&T UNIX. In fact, the last releases of BSD from Berkeley still contained substantial amounts of AT&T code. They made the Net/2 release without AT&T code, but this was not a complete system. 386BSD reimplemented most of the missing parts and modern BSDs built on that work.
tl;dr: while modern BSD does not contain AT&T UNIX code anymore, it evolved from AT&T UNIX. In contrast to Linux, it was not a clean-room implementation.
The point is that it is not a clone of UNIX, it's one of the evolutionary strains of UNIX. You probably won't find substantial amounts of AT&T Research UNIX in Solaris, but it's still UNIX. The lineages of BSD and Solaris can be traced back directly to AT&T Research UNIX.
tl;dr: while modern BSD does not contain AT&T UNIX code anymore, it evolved from AT&T UNIX. In contrast to Linux, it was not a clean-room implementation.