I believe they are referring to proxying requests to third parties, rather than the first part of the post which refers to proxying to your own backend. Proxyi

I believe they are referring to proxying requests to third parties, rather than the first part of the post which refers to proxying to your own backend. Proxying requests to a third-party will mean browser security features like CORS will assume they are entirely safe due to them appearing to come from your domain, and not “gist.github.com” etc.