This was indeed one of Davis-Secord's main arguments.

The term “Anglo-Saxon” was rarely used at the time in England. They did not see themselves as a unified race, and actually were a motley collection of different peoples competing with each other. The kingdoms shared a language now known as Old English, but they spoke different dialects, warred with each other and sometimes allied with native forces to get an edge over each other.

The “Anglo-Saxon” label first appeared shortly before 800 CE in continental Latin works as a way to distinguish the English speakers in England from the distant relatives they had left in what are now Germany and Denmark. The label had not yet developed its overtly racist connotations at this point, but it nonetheless distinguished peoples in a way that built into modern racism. The majority of the term’s appearances in early documents within England itself occurred again in Latin texts, where it indicated expanded royal control over the previously separate kingdoms of the Angles, the Saxons and several other segments of the island’s population. Specifically, King Aethelstan (d. 939 CE) was described in charters at the end of his life as “emperor of the Anglo-Saxons and Northumbrians, governor of the pagans and defender of the Britons.” Those Angles and Saxons — and their kingdoms — were distinct, and the other kingdoms of English speakers (not to mention Wales, Scotland or Danish settlements) were certainly left out of the term’s coverage.

This is as ridiculous an argument as saying that, because Alfonso VII titled himself 'Emperor of all Spain' but maintained Castile, Leon and Galicia as three separate kingdoms in personal union under him, there were no Spanish people in 1150. Actually even worse, England was a single unitary kingdom after Athelstan and never partitioned between his successors unlike Alfonso's Spain from two centuries later.

(Also, in that passage 'Northumbrians' referred to the Anglo-Danes of northern England, where Viking influence had been strongest and the longest-lasting Viking kingdom in England was located. Ethnic divisions between Angles, Jutes and Saxons had already effectively ceased to exist several centuries before Athelstan, hence why - as the article admits - he literally called himself emperor of the Anglo-Saxons and not emperor of the Angles and Saxons)