CodjiFlo[1,2] - a code review tool inspired by Microsoft's CodeFlow, used by ~40,000 developers. It is especially tailored to power users of pull requests to improve contextual understanding and ease of code review and collaboration.
Thanks for checking it out. It should be possible in the future, I haven't explored what GitHub APIs are available for logged out users. It's not my main focus now because I want to nail the capabilities for repo contributors, who do code reviews most often and will need be authenticated to post comments.
[1] https://github.com/pedropaulovc/codjiflo/
[2] https://codjiflo.vza.net