Why did I receive an email from Intuit (yes, it was from them) telling me I needed to supply my 2016 AGI when I had already been told that the IRS had accepted my return?

If you have received notification that your return was accepted (not just sent, but accepted), then you can ignore that email.  You can check the status of your e-file at the link below -

 

 

As for the email, it was likely generated in error, but you should be safe and not click on any links inside that email and please report to https://security.intuit.com/.