Earlier, qasr was studied as restriction or exclusivity, as in «إِيَّاكَ نَعْبُدُ», where worship is restricted to God.
In ijaz al-qasr, the word qasr refers to concision, not necessarily to exclusivity. «وَلَكُمْ فِي الْقِصَاصِ حَيَاةٌ» has few words with extensive implications.
The shared term must not hide the distinction: restrictive qasr concerns exclusivity, whereas ijaz al-qasr concerns condensed yet sufficiently meaningful wording.