لَمْ يَغُرُّوكُمْ غُرُورًا وَلَكِنْ رَفَعَ الْآلُ شَخْصَهُمْ وَالضَّحَاءُ
Meaning: They did not catch you by a deceptive surprise; rather, the mirage in the bright forenoon made their figures rise into view.
Al-āl here is a heat mirage, not a family or dynasty. Shakhṣahum denotes their visible figures; al-ḍaḥāʾ is the brighter part of the day.
The contrast lam ... wa-lākin rejects the idea of surprise and supplies a visual explanation: the approaching force could be seen across the shimmering plain. Natural imagery serves a persuasive claim.