He's referring to European cultures/civilizations that ended when they hired large numbers of mercenaries to supplement their own flagging forces and wound up being assimilated into the mercenary populations culture.
Nah if we're going by mercenaries then the list is even longer. Carthaginians were nearly overthrown by theirs, Romans likewise (and were by their barbarian auxiliaries), the Thebens usurped by Macedonian mercenaries (who would eventually conquer the world) or when the Swiss got all cheeki in Italy.
I'm referring to cases where powerful peoples have foolishly tried to use foreigners for their own gain only to have them settle and take over their land.
The Anglo-Saxon one for example actually starts with Roman Britain (or rather, immediate post-Roman Britain) where the tyrant Votigern invited Saxons to settle all along the English coast in order to ward off seaborne raiders from Scotland. This worked very well up until three of the Saxons rebelled and called upon others of their race to fight with them and they did. Native resistance would continue and from that would rise the figure known only as Arthur who may have been several people or one person or so on. The real migration however would begin when some Breton King (unknown) invited Angles, Jutes, Saxons, Franks and Frisians to his Kingdom to once more ward off the Picts in Scotland. People of Cornwall and Wales have very unique cultural traditions compared to the rest of the southern Isles because they were not conquered by the Anglo-Saxons or impacted much by their settlements.
The Seljuk Turks invaded Anatolia but they were of the Sultanate of Rum, not the Ottoman Empire, and contained in Anatolia. The Ottoman Turks were themselves brought to Anatolia from Turkmenistan by the Sultanate of Rum, but after the Sultanate of Rum grew too weak one great big Turkish state became many small ones. Around this time the Byzantines in the West are in piss poor shape and Emperor John VI finds himself facing a civil war and Serbian encroachment, so he hires the Ottomans as mercenaries to fight for him. The Ottomans do fight for him and then continue fighting. When they defeat the Serbians and rebels they don't go home and remain in the Balkans. It becomes their land ^_^
In Mexico's case you first have them being filled with Spaniard mixes after all the central Americans were assimilated/conquered/killed by foreign diseases and so on. The province of Texas was quite sparsely populated and vulnerable to Native American or American raids, foreign agents agitating for rebellion and the Mexican government with its pennies stretched thin didn't think they could afford to pacify the area themselves. So they liberalized their immigration policy and allowed immigrants from the USA to enter. They ended up getting loads of southern Americans who were protestant and very distrustful of Catholics, in a country that was officially Catholic. When the Americans outnumbered the Mexicans in the province the Mexican government panicked and made American immigration illegal, but the Americans still came anyways. The Mexican government were quite suspicious too of the Americans, suspecting them of moving to secession.
By the time the Texan revolution was underway the impact of American immigration to Mexico going by Texan figures alone would amount to about 0.2% of Mexico's population being American. But in Texas itself the impact was far more severe, with Americans making up 79% of the population. Locally, the area was already lost before shots were fired :
D