I've heard that the nation of Yajuj and Majuj (Gog and Magog) is in the ratio 999:1 to humans, meaning there are 999 of them for every human. There are approximately 7.8 billion humans on the earth today. This would mean there are approximately 7792.2 billion of the Yajuj and Majuj, which is a number beyond human comprehension. It seems impossible for such a large group to remain hidden and undiscovered for so long. So are the Yajuj and Majuj really on earth or are they in a completely different place? If they are on earth, how is this possible?
“Bring me sheets of iron" - until, when he had leveled [them] between the two mountain walls, he said, "Blow [with bellows]," until when he had made it [like] fire, he said, "Bring me, that I may pour over it molten copper." - (The Holy Qur’an 18:96)
When Zulqarnain built the wall he could only build it in one place, the earth. This quote signifies that as he got people who he stumbled across to bring him these materials in which the wall was made. The location is unknown, many come up with theories but ignore them as they are merely theories. They are hidden for a reason and they will be released closer to the day of judgement. Although I can understand why you think that but there are many other hadiths which further explain the events of yajuj and majuj.