What is the meaning of "he will be resurrected on the Day of Judgment as a nation by himself," mentioned in the Hadith about Zayd?

Imam Ahmad transmitted in his Musnad, from Nufayl ibn Hisham, from his father, from his grandfather, that the Messenger of Allah (peace be upon him) and Zayd ibn Harithah invited Zayd ibn 'Amr ibn Nufayl to a meal. He (Zayd ibn 'Amr) said: "Indeed, I do not eat from what has been slaughtered upon idols." After that, the Prophet (peace be upon him) was never seen eating anything that had been slaughtered upon idols.

And when the Prophet (peace be upon him) was asked to seek forgiveness for him, he said: "Yes, I will seek forgiveness for him, for he will be resurrected on the Day of Resurrection as a single nation (ummah wāḥidah)."

As for the meaning of "a single nation (ummah wāḥidah)," it has been said: 1. That he was unique in a religion in which no one shared with him. 2. That he stood in place of a community in his servitude to Allah, similar to their saying, "So-and-so is a tribe in himself."