The Plus model would only be needed for the big battery - like the 12 mini the smaller battery with 5g might reduce battery life to unacceptable levels.Very possible. The rumor also suggested the processor will get an upgrade. Not sure this will require a "Plus" model, but we'll see. Either way, we are talking about retaining a legacy "Home Button" design for now while saving the XR-based form factor for a future iteration.
Look at the following battery sizes:
iPhone 8 Plus: 2691 mAh
iPhone SE 2020: 1821 mAh
iPhone XR: 2942 mAh
iPhone 12 mini: 2227 mAh
iPhone 12: 2815 mAh
So an 8 Plus sized iPhone would have battery life close to the 12 - I believe it had a higher density 5.5" screen too.