It's based on an interpretation of "derived from".

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.