zeropointreference:
You first need to place the Skylight outside the Roof (see warning below) so it is not associated with the Roof and does not use its slope. Then under Parameters the
Roof Pitch is available, and can be set to 0. I did not look at the code of the object, but my guess is that when the Skylight is associated to a Roof its slope is tied to the Roof and so cannot be changed independently.
David