"And they say, 'Exalted is our Lord! Indeed, the promise of our Lord has been fulfilled,'" meaning: It has already been fulfilled.
Muhammad said: The meaning is: The promise of our Lord was fulfilled, and the particle "in" (verily/if) and the lam (l) were added for emphasis.
﴿وَيَقُولُونَ سُبْحَانَ رَبِّنَا إِنْ كَانَ وعد رَبنَا لمفعولا﴾ أَيْ: قَدْ كَانَ.
قَالَ مُحَمَّدٌ: الْمَعْنَى: كَانَ وَعْدُ رَبِّنَا مَفْعُولًا، وَدخلت (إِن) وَاللَّام للتوكيد.
