Is output pin set to LOW grounded
This will effectively connect the pin to ground. Note that the connection is
made by a transistor that has a finite resistance. This output resistance is
not really specified, but from the curves on the “typical characteristics”
section of the datasheet you can infer that the typical value is about
26&nbsp;Ω.

The only hard specification states that the OUTPUT LOW voltage
should be no larger than 0.9&nbsp;V, as long as the pin is not sinking
more than 20&nbsp;mA. C.f. the table “DC characteristics” of the
ATmega2560
datasheet.