Interfacing two ICs with different thresholds
The ADuM1251 has an output low level of 600 - 900 mV when sinking 3 mA.
The ATxmega32A4U has an input low level of 0.5 - 0.3×VCC (990mV).   

The ATxmega32A4U has an output low level of 0 - 400 mV at 3 mA. (page 113)
The ADuM1251 has an input low level of 500 - 700 mV when sinking 3 mA.  

Both seem compatible to me. Except maybe the italic numbers, but you have very bad luck if that takes you down.

Note: I should add that the sides on the ADuM1251 are not identical.