"And those before them denied," meaning those before your people, O Muhammad; that is, from the nations of the past who were of your lineage.
"And they have not attained a tenth," meaning they have not attained a tenth "of what We have given them" of worldly possessions; that is, the nations of the past.
﴿وَكَذَّبَ الَّذِينَ مِن قَبْلِهِمْ﴾ من قبل قَوْمك يَا مُحَمَّد؛ يَعْنِي: مِنْ أُهْلِكَ مِنَ الْأُمَمِ السالفة.
﴿وَمَا بَلَغُوا مِعْشَارَ﴾ مَا بلغ هَؤُلّاءِ معشار؛ أَي: عشر ﴿مَا آتَيْنَاهُمْ﴾ من الدُّنْيَا؛ يَعْنِي: الْأُمَم السالفة.