In paragraph 1.5 “Line Schape”, there is stated:
It has to do with one of the selection rules in EPR, namely that only the magnetic moments from the sample in the direction of the external field (to be more precise: perpendicular to the direction of the magnetic field created by the microwaves) are detected.
I would think that magnetic moments from the sample that are not perpendicular to the microwaves would also be detected, albeit not that easily as magnetic moments from the sample that are perpendicular to the microwaves. I do not expect this selection rule to be so strict.