Sorry for the late response. I mentioned Freedesktop in the context of the Linux desktop. Freedesktop is responsible for the desktop ecosystem typically associated with Linux, and it’s what application developers refer to when discussing Linux compatibility. Steam for ChromeOS is not referred as Steam for Linux for a reason, it’s an entirely different application. Of course, there are exceptions, such as NixOS and Alpine for example, but both of them still aim to be part of the “Linux desktop” ecosystem, whereas ChromeOS does not, and yes, it’s “Linux” under the GUI stack, but referring to ChromeOS as a Linux desktop would only confuse those less familiar with the topic or worse, mislead them.

Generally, Linux as a term refers to the Freedesktop standards, Linux kernel, and GNU userland. ChromeOS fulfills the latter two, but it lacks the Freedesktop standards, which are an essential part of the Linux platform. Therefore, it is logical to distinguish ChromeOS from standard Linux distributions since it diverges significantly from them. This differentiation has nothing to do with the Terminal, as it isn’t necessary to use it at all on most distributions. Elementary OS and Endless OS adhere to the Freedesktop standards, they are parts of the same platform as other Linux distributions.

I meant that most major distributions had KDE available for those who were bothered by the lack of thumbnails.

Only the GNOME/GTK file picker did not display thumbnails for a while, but this functionality has now been available in all file pickers for some time.

Not so much a fork, rather a hardened Firefox, like Librewolf and Arkenfox on desktop.

Mostly from search engines, especially Google. They also own Mozilla Corporation, a for-profit entity which reinvests all of its profits back into the Mozilla Foundation.