The plan was always to offer it with the 7.3L and the 6.7L. I know this because at work we talked about the different aftertreatment system on the F-600 compared to the rest of the chassis cab lineup. Also this: https://www.worktruckonline.com/331375/ford-all-new-f-600-rounds-out-super-duty-lineup. I wonder if it got pushed to the 2021MY.
Also, some customers will find the F-600 more attractive than the F-650 due to its smaller size and the availability of 4x4. If you haven't driven an F-650/750, they are extremely unwieldy compared to an F-550/600.