Hmmm, I hadn't visited Fast.com in some time. Just visited, and it quite clearly states "Your Internet Speed" and the title of the page visible in the tab is "Internet Speed Test". I can't imagine why people would be consfused. /s
I clicked around to see if they defined it in more detail about specifically testing Netflix CDN vs general internet activity, but I found not such information. However, this has always been understood on my part to be the case. Maybe because of where I was working when it first came on scene? I clearly didn't get that information from their website.
This is true of most internet speed tests though. Even speedtest.net uses their own servers. If you're measuring for speed you just need to be sure that the server has higher bandwidth available than your ISP.
If you're looking for a generic internet connectivity test then you'll need to test a few dozen sites at least, with diversity in which CDN's they're using and so on.
I clicked around to see if they defined it in more detail about specifically testing Netflix CDN vs general internet activity, but I found not such information. However, this has always been understood on my part to be the case. Maybe because of where I was working when it first came on scene? I clearly didn't get that information from their website.