I don't think there's a way to do this under Groups.io because the actual sender's address is not a part of the From: field.
If it were, you could use the standard Reply keyboard shortcut, but doing that for a list message replies to the group, not the originating sender. I believe the only way to reply directly to sender, without also including the list, with ease in most e-mail clients is hitting that Reply to Sender link at the end of messages or digest entries. That's why that particular link came into existence in the very early days of Groups.io.
I've never dealt with an e-mailing list where direct replies to originating senders of single messages didn't involve a workaround of some kind as the From: and Reply-To are usually set to the mailing list address.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Growth for the sake of growth is the ideology of the cancer cell.
~ Edward Abbey