It does not matter if it's in the GPL explicitly or not since we're talking about uutils and their stance on it, and they've written that:
https://github.com/uutils/coreutils/blob/6b8a5a15b4f077f8609...
> we cannot accept any changes based on the GNU source code [..]. It is however possible to look at other implementations under a BSD or MIT license like Apple's implementation or OpenBSD.
The wording of that clearly implies that you should not look at GNU source code in order to contribute to uutils.
