You are correct, Randall. Webinar hosts may not register for their own webinar.
A nice way to get around this if you have gmail (or your company does) is to append a plus sign and then another word to your username, for example, firstname.lastname@example.org.
If you look in the details for that specific activity you will see:
"Error registering with webinar provider. exception='API Error: Webinar host can not register for the webinar."