Thanks for confirming this! This all makes a lot of sense, but might merit clarifying in your documentation.
When I see sample code, I generally assume that I can copy and paste it onto my own site. So a comment "You can try this code on OUR site, but if you want to use it on YOURS, you need to take care of your own companion hosting" in your sample code would be helpful.
Having read your site, I was certainly aware of all the companion options you enumerated above. However, I did not see your above sentence "companion.uppy.io is really only meant for demo purposes and hence throws CORS errors if you'd try to use it for your own website" (or anything equivalent) anywhere on the site.
I realize that this sentence (while clear, accurate, and quite reasonable) takes a bit of polish to turn into a positive message which won't scare away potential users ;-)
Alternatively, maybe you could set up your demo server to accept requests from randos for exploratory purposes, but with a quota set low enough that it won't be abused for production?
When I see sample code, I generally assume that I can copy and paste it onto my own site. So a comment "You can try this code on OUR site, but if you want to use it on YOURS, you need to take care of your own companion hosting" in your sample code would be helpful.