When there is a comment on a pull request and the email is set out about the comment, it says "View this pull request or add a comment by replying to this email. "
Replying to the email did not put anything on the website.

To be fair, I did only wait 10 minutes before submitting this bug, but I would have expected it to work within that time.

I have not tried to test it again, but can if needed.

I sent my email using the gmail web UI.