You can also select all (ctrl-a) create a new email and paste everything in. Even simpler, look at your email on something other than Outlook on a Windows machine - this add-in is absolutely not for protecting intellectual property! I did contemplate
making it aware of only reply/replyall and not forwarding but it seemed silly to not support that since it uses exactly the same mechanism as reply/replyall. Forward as attachment, though, uses a different mechanism and not one I've investigated how to disable.
So, sorry, I have no plans to prevent forward as attachment.
Fwiw, IRM is the tool to use to take much more robust control over who can view, forward or copy email items.