With incognito mode data is still fetched client-side: the browser still has to send requests directly to the target platform. Plus the origin platform will still set session cookies.

Instead PR (and as far as I know all other alt-frontends) fetches data and does scraping server-side, so that the browser never contacts the platform directly.

The idea is that you can host these alt-frontends wherever you wants (eg. on a remote VPS) outside of your network and they deal with the platforms as intermediaries, and people just have to open the proxy website and browse for content…