I was wondering why this would be. The comment for the "Allow frontend edit" clearly states it's for making it editable in the user's account page, and this is how the Magento 1 version of this plugin works. However, I installed it today on Magento 2 (this was my first 3rd party module installed on Magento 2), and I noticed the register.phtml even has a condition for this:
if ($_username->isEnabled() && $_username->isEditable()):
So, having it disabled completely blocks it from being able to be set at registration. I'm not sure if this was intended, and it would be easy for me to adjust it if this was the intention. But was it?