The GNU coreutils are a reimplementation of the Unix utilities. With some caveats, the same tools are available on BSD and Solaris derivatives which are both Unix.
So while GNU is obviously an important project, it would not be correct to say "xargs" is not a Unix tool.
I and many others take “Unix” to mean any UNIX or UNIX-like operating system, whereas “UNIX” with all capital letters means only the ones that are certified UNIX.
For example the initial implementation of AWK was in 1977 [1], a few years before GNU even existed [2], so it _is_ a Unix tool.
[1] https://en.wikipedia.org/wiki/AWK#History [2] https://en.wikipedia.org/wiki/GNU#History