I just went through the registration and it's basically a replacement for the disembarkation cards (customs/immigration) handed to you on the airplane prior to landing. You complete the registration, and it gives you a QR code for immigration/customs to scan with all the details that you'd normally handwrite on the cards.

So, unless they are either getting rid of the vax/test requirement by 11/14 (unlikely), or they are coming out with a new app in the next few weeks that combines them (possible), then you still need to use both.

EDIT: I was just told that after 11/14 this will replace MySOS completely, but the new version with vaccine info will be available after 11/1.