Delta is inconsistent in some of their marketing materials, so for different purposes Comfort+ has more or fewer traits of a different "Cabin" than Main Cabin. Delta tells the governments of the world that Comfort+ is not a different cabin (marketing it as an "experience" in the Main Cabin), because if it was they would have to to pay downgrade compensation under laws like EC261 when they move people from Comfort+ to Main Cabin.
However, my gut tells me that this change is intended to move paid Comfort+ up above Main Cabin in the upgrade priority for two reasons: (1) Delta wins if they can convince some PM/DM travelers that currently book Main Cabin and get the instant upgrade to Comfort+ to pay for Comfort+ instead (likely because it's within their corporate policy) and (2) DL tries to find value-adds to differentiate Comfort+ from Main Cabin, and enhanced upgrade priority is a value-add.
I can confirm that the UK air passenger duty is the same for Main Cabin as it is for Comfort+ - but Premium Select pays a premium.