“Lā taṭluʿ” expresses a wish not merely because it is brief, but because it addresses dawn and asks for a natural event outside human control to be suspended.
لَا تُغْلِقِ الْبَابَ
Do not close the door.
Addressed to someone who can close it, this can be a literal prohibition or a request among equals.
يَا صُبْحُ لَا تَطْلُعْ
O morning, do not rise.
The arrival of dawn is beyond the speaker’s control; wishing predominates. Still, impossibility alone does not settle every case without context: longing, regret, or other expressive purposes can overlap.