I’m not entirely sure and haven’t had a chance to test on an android device. But I think it kind of goes beyond this use case. Even if you’re on a desktop and you just happen to have not engaged in the community in a while it would encourage engagement because it would auto log you in when you went to respond.
Generally I’ve disabled reply by email on most discourse instances we manage because the result of the email parsing can be a bit hit or miss and the mobile web interface for disqus is so great.. so when it works (you’re already logged in) clicking on the “visit topic” (I might even add a reply now button that opens up the reply dialogue) is almost as seamless as hitting reply in your email client.