The trouble is, GNU is only _part_ of the GNU/Linux operating system. What people actually experience as "the operating system" day to day includes the kernel, and a bunch of GNU userspace, and a bunch of BSD userspace, and a bunch of completely unrelated software. Is systemd a GNU project? Is yum/apt/dnf/whatever? Is KDE or XFCE or X.org?
They're totally right that the GNU components are an indispensable part of a Linux system. But if they wanted to guarantee they got continuous credit for their components over the decades then maybe GNU shouldn't have given their software away to the world to do with largely as it pleases.