A speaker may know the identity of a referent and still use an indefinite expression for rhetorical effect. Someone saying «جَاءَ رَجُلٌ» may know the man's name but presently wishes to report the event of arrival rather than recall his established identity.
Likewise, the covering in the Qur'anic example may be presented in order to evoke its special kind or force. Grammatical indefiniteness does not by itself prove ignorance in the speaker's mind.
The interpretive procedure is to establish what the noun refers to and then ask which feature of the communicative situation makes an indefinite form appropriate.