«وَهُمُ الْمُخَضْرَمُونَ الَّذِينَ أَدْرَكُوا الْجَاهِلِيَّةَ وَالْإِسْلَامَ وَلَمْ يَرَوُا النَّبِيَّ ﷺ»
The mukhāḍramūn lived through both the pre-Islamic and Islamic periods but did not see the Prophet ﷺ.
Chronologically they stand extremely close to the Companions. Yet the absence of an established qualifying encounter means they cannot all simply be classified as Companions. Their transmissions may concern events of the Prophetic era, but the actual route by which they learned of those events remains important.
Illustration: Someone lived before Islam, witnessed the Prophetic period, embraced Islam, but did not meet the Prophet. Contemporaneity with the Companions does not by itself constitute technical companionship.