You can't reply to the whole digest email, but it looks like you have set it up so you can reply to a specific message in the digest.  I clicked on the Reply to this Message. in the email and it works as long as you are signed in to Groups..io on the web.
If you are not, you don't see the "groups reply" button.
It would be useful if it brought you to a login screen if you weren't already logged in, then you can login and then go to the specific message. Each message does have its own URL, so this should be possible!