Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
It is well known among genealogists and historians that the Romans are descendants of Esau (al-'Is), son of Isaac, son of Abraham, peace be upon them. Therefore, they are described as being from the descendants of Isaac. This does not mean that all descendants of Isaac are Romans; rather, there may be other nations and peoples who trace their lineage back to Isaac, peace be upon him.
As for the aforementioned hadith, Imam Muslim narrated it in his Sahih, in which the Prophet ﷺ said:
“The Hour will not be established until seventy thousand from the descendants of Isaac invade it.”
He then mentioned the conquest of a great city with the proclamation of Allah's greatness, and that they would enter it and take spoils. Then news would reach them that the Dajjal had emerged, so they would return.
Thus, the hadith is authentic and firmly established, and it is not correct to reject it merely because of difficulty in understanding the wording “the descendants of Isaac.”
Some scholars wondered how the conquerors of the city could be from the descendants of Isaac when the city itself was among the lands of the Romans, while the Romans themselves are descendants of Isaac. The scholars have answered this in several ways, the most important of which are:
The Romans may enter Islam at the end of time. It is authentically established in Sahih Muslim that there will be fighting and major battles between the Romans and the Muslims, which indicates that some of them may embrace Islam.
The fact that the Romans are descendants of Isaac does not mean that all descendants of Isaac are Romans. There may be other groups who trace their lineage to Isaac, peace be upon him, and are Muslims.
The hadith does not say that the entire army will consist of descendants of Isaac. Rather, it specifically mentions seventy thousand from the descendants of Isaac, which does not prevent other Muslims from being with them.
Some scholars have offered other interpretations of the wording “the descendants of Isaac,” but there is no need to reject the hadith when it can be understood in these ways.
Therefore, the more likely view is that the Romans are descendants of Esau, son of Isaac, peace be upon him, and that the hadith concerning “seventy thousand from the descendants of Isaac” is authentic. There is no contradiction between this and the fact that the city they conquer is in the lands of the Romans, because the descendants of Isaac constitute a broader group than the Romans, and because some Romans may embrace Islam and become part of the Muslim armies at the end of time.
And Allah knows best.