How much GNU is there in GNU/Linux?

Quite so. This is the logic behind the FSF's adoption of things like the X Window System and a bunch of BSD utilities -- and, indeed, the Linux kernel itself -- under the GNU aegis: that they are freely available and necessary to produce a free operating system, thus we indicate that they are part of 'the GNU system', i.e., what we would now call a distribution. (You'll notice that this distribution is purely notional: there is nowhere you could download a running 'GNU system' from, you have to assemble it out of pieces)

Where the FSF goes haywire is that it considers that its distribution is a fundamental property of the programs that make it up, and therefore also of other distributions made out of the same pieces. You could as easily say that coreutils should be called RedHat/GNU coreutils because of the longtime employer of its maintainer: by extension, because RH maintains so much of the software that goes into Linux, Ubuntu should be called RedHat/Ubuntu. This is plainly mad, and RH would fight it because it's trademark dilution as well. (You could argue that coreutils's copyright is assigned to the FSF so it should be allowed to keep other entities out of its name, but X's copyright *isn't* assigned to the FSF yet the FSF feels happy to call it part of the GNU system. So copyright assignment cannot be relevant here.)

Here, as in many ways, RMS and the FSF are stuck in the past: in the era they come from, and in the proprietary world now, products really *did* get dubbed with the name of the vendor who was distributing them. The free software world stopped working like that fifteen years or more ago, but the FSF never changed its ways here. Product names are, more than ever, arbitrary monikers, not credit lists: but the FSF doesn't seem to have noticed.