The Prophet (peace and blessings of Allaah be upon him) said: Whoever claims to belong to anyone other than his father on him will be the curse of Allaah and the angels and all the people.
(Reported by Ibn Maajah, 2599; Saheeh al-Jaami, 6104).
You keep on bring up this hadith, but there is a difference between nationality ethnicity, race and lineage.
Why is it that there are Saudi Arabian with different ethnic origins?
There are Hijazi Saudis with Turkish lineage, African and Yemeni lineage.
If you are son of Abdullah you will always be son of Abdullah. Yet the sons of Abdullah can migrate to a different nation and become part of said nation after many generations.
Our ancestors understood this, which is why they had no problem believing that their forefathers were of Arab origin, despite now being Somali.