Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Here is the Hadith mentioned in the question:
Abu Zar reported that Allah's Messenger (Sallallahu Alaihi wa Sallam) said: "You will conquer Egypt, a land where Qirat (a measure of weight and area) is used. When you conquer that land, you have to treat its people kindly since they have a right of kinship upon you. And when you see two persons fighting for the space of a brick, then get out of that. He (AbuDharr) then happened to pass by Rabi'ah and AbdurRahman, the two sons of Shurahbil ibn Hasana, and they had been disputing for the space of a brick. So he left the land". [Reported by Imams Muslim Ahmad]
The Prophetic order concerning leaving Egypt is, in fact, special to Abu Zar (Radiya Allahu Anhu), to whom the Hadith is addressed. This deduction is based on:
1) It is unreasonable that Allah, The Great and Almighty, orders Muslims to fight Jihad in His cause to spread Islam, announce His words, and save people from misguidance to right path and from darkness to light, then, He re-orders Muslims in general to leave the conquered land for which they have offered their souls and wealth, especially after raising the Banners of Islam in that land. In other words this is contrary to the broad purposes and goals of Sharia.
2) The majority of Muslim scholars say that if an order or a speech is addressed to some specific person, others will be excluded. In simple words, any ruling in such an address is special for the addressee only. But, if there is any other Hadith or juristic reasoning by analogy that indicates both the addressee and others, then there will be difference, i.e. all parties will be subject to such a ruling or order. As clarified above, the Hadith mentioned at the beginning is a special order to Abu Zar.
And the Prophet (Sallallahu Alaihi wa Sallam) ordered him to do so to help Abu Zar avoid disorders and ordeals. This Prophetic order was a result to some issue Allah let the Prophet (Sallallahu Alaihi wa Sallam) know about. This issue may be an introduction to the Prophet (Sallallahu Alaihi wa Sallam) Hadith that state that: "Abu Zar will expire in a wild land". [Ahmad]
Another evidence in this vein is: The Hadith urges Muslims, all Muslims to treat the people of Egypt kindly, while it urges Abu Zar, by name, to leave it.
Allah knows best.