Yes, presumably this behaviour is intentional because otherwise it’d be counter intuitive for new or inexperienced years.
I think the GUI could be improved by the addition of a comment, in the box you took a screenshot of, stating that if there is no internet access then any email address can be used providing it conforms with the generally accepted syntax (e.g. firstname.lastname@example.org)
There should be no problem with decrypting your data if AxCrypt cease business because the software is fairly robust but it’s always best to check and would have saved you a lot of heartache if you’d done the same with the now defunct Cloudfogger.
It’s always worth burning a copy of AxCrypt to DVD and USB as well as uploading it to your cloud storage provider should you ever need a copy of the software in an emergency and this website is down.