With this discussion on position, I checked the roster on the Celtics "official" website and they had players categorized with the following positions:
G      Smart, Brogdon, White, Pritchard, Thomas, Davison
G-F   Brown
F-G   Tatum
F       Gallinari, Grant, Hauser
C-F    RWill, Horford, Kornet
C       Kabengele
Both Horford and RWill are listed as C-F.  Since Brown is G-F and Tatum is F-C I am guessing this implies that Brown is more G than F and that Tatum is more F than G (which makes sense).  By that logic, they have both Horford and RWill as more C than F so if Hoopshype followed that, they would list them as Centers.  Apparently we don't have any F-C (I would say that this would apply to Gallinari for example).
One thing that stands out when you see it this way is we have a lot of Guards.