Indefiniteness can also convey contempt or diminution of status. An excuse called merely «عُذْرٌ» could, in an appropriate context, be presented as one hardly worth considering.
The grammatical category has not changed: it is still an indefinite noun. What changes is the effect of the speaker's attitude and the discursive situation. For that reason the same surface form may be dignifying in one context and disparaging in another.
A sound analysis does not declare every indefinite noun contemptuous; it specifies the contextual evidence and explains how the listener infers the disparaging sense.