وَلَئِنْ أَخَّرْنَا عَنْهُمُ ٱلْعَذَابَ إِلَىٰٓ أُمَّةٍ مَّعْدُودَةٍ لَّيَقُولُنَّ مَا يَحْبِسُهُۥٓ أَلَا يَوْمَ يَأْتِيهِمْ لَيْسَ مَصْرُوفًا عَنْهُمْ وَحَاقَ بِهِم مَّا كَانُوا۟ بِهِۦ يَسْتَهْزِءُونَ
wa la-in akhkharnā ʿanhumu'l-ʿadhāba ilā ummatin maʿdūdatin la-yaqūlunna mā yaḥbisuh, alā yawma yaʾtīhim laysa maṣrūfan ʿanhum wa ḥāqa bihim mā kānū bihī yastahziʾūn
"If We defer the punishment from them to a numbered span, they will certainly say, 'What is holding it back?' Take note: on the day it comes to them it will not be turned away from them; and what they mocked will close in upon them."
The sense of the word beyond "community" was treated at al-Zukhruf 43/22, where the sense of umma as "the road followed, the line kept to" was tabulated. I rely on that treatment.
Here the word comes in a third sense: an interval of time, a span. The idea of "priority, being at the source" in the root أ-م-م reaches also to the countability of a stretch of time. And the adjective confirms it: maʿdūda — "numbered".
The sentence therefore reports not an indefiniteness but a definiteness: there is a deferral, but the span is numbered. And the sūra will come back to this word: in the hundred and fourth verse, wa mā nuʾakhkhiruhū illā li-ajalin maʿdūd — exactly the same idea.
The kind of question must be recorded, and I give it as an observation: the question does not dispute the reality of the punishment; it asks about its delay. The mockery, that is, is aimed not at the content of the claim but at its calendar.
The demand that the punishment be hastened has been treated in many places across this series (al-Raʿd 13/6, Sabaʾ 34/29, al-ʿAnkabūt 29/53, al-Ṣāffāt 37/176). I rely on that treatment.
The root ح-ي-ق: to come down around a thing and enclose it, to encircle. The root was treated at Ghāfir (al-Muʾmin) 40/45 and al-Aḥqāf 46/26.
And note the mood of the verb: ḥāqa is a perfect — something that will happen in the future is told as if it had happened. In Arabic this reports certainty.