You are correct.  If you entered the "Filing interview" that indicates you intend to file your tax return.  Filing your tax return does not change the due date for your payment.  It is still 15 Apr 2021.  You do not have to send the payment and return via the same method or at the same time.  You will just mail in your payment on or before that 15 Apr deadline to avoid any late fees or interest.    

For future reference, if you want to file an extension or any other form instead of your tax return, you would find that in the "Other Tax Situations" tab, under the "Other Tax Forms" topic.

This is the third year that I wanted to file for an extension.  This is the third year that I found your platform not helpful.  This year was the worst.  There is no option easily located to do this and also rifle your state tax.  This should be easy but your platform makes it impossible. I chose to print emy federal tax and efilestate, but it did not give me the option