وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لِلَّذِينَ ءَامَنُوا۟ لَوْ كَانَ خَيْرًا مَّا سَبَقُونَآ إِلَيْهِ
"Those who disbelieve said of those who believe: had it been anything good, they would not have reached it before us."
Its structure is this: what is good comes first to the one who is superior. If these people received it before us, then it is not something good. That is, the measure is not what has come but who received it.
I record this as my own reading: the objection is not an item of evidence but an assumption about rank. And this assumption falls under neither of the two kinds of evidence asked for in the fourth verse (observation and transmitted report).
Other examples of this stance have been treated in this commentary: in ʿAbasa, the turning towards the man of standing while the blind man who came was left unattended; in al-Humaza, the amassing of wealth being taken for superiority. The shared point: reading worth off the position of the one who carries it.
وَإِذْ لَمْ يَهْتَدُوا۟ بِهِۦ فَسَيَقُولُونَ هَٰذَآ إِفْكٌ قَدِيمٌ — "since they have not found guidance by it, they will say 'this is an old fabrication'."
إِفْك — root أ-ف-ك: to turn a thing away from its direction, to turn it inside out. That is, the word used for "fabrication" means "turned about". The word will come back in the twenty-eighth verse, and there it will describe their own condition: wa dhālika ifkuhum — "that was their fabrication."