I notice this website refers to Trisquel as a distribution of "GNU/Linux" and not "Linux". Why?

There is a long-standing naming controversy. Most people who use the system today don't know that what they're actually using is the GNU system combined with the kernel Linux. For many years, the media and the user community itself has given undue weight to the contributions that come from Linus Torvalds' camp and fostered a skewed account of the operating system's history, while barely acknowledging the existence of the GNU project at all. The GNU project was started in 1984 by Richard Stallman to develop a complete free operating system, because none existed at the time. Its design closely followed that of Unix because Unix was highly machine-portable and (at that time) pervasive. Linus Torvalds did not write a whole operating system, he only wrote the last missing piece, a kernel, and he only did that in the first place because development of Hurd, the GNU project's own kernel, was lagging behind (and has not been completed to this day). Torvalds didn't write the kernel because of a belief in "open source", a term that wasn't even coined until 1998 and misses the point of free software, and he originally released it in 1991 under a proprietary license until he was persuaded to relicense it under the GPL the next year. Saying "GNU/Linux" instead of "Linux" is fairer and more accurate. Without the irreplaceable software contributed by the GNU project − and even more importantly, the founding ideas of freedom − the system most mistakenly call "Linux" would not exist. It's that simple.