The question is not asking what the Anglo-Saxons were, since they were a people. The question is asking which people they perceived themselves as, for which the

The question is not asking what the Anglo-Saxons were, since they were a people. The question is asking which people they perceived themselves as, for which the correct interrogative pronoun is "who". The question is well written, although it could also be phrased as "How did the Anglo-Saxons see themselves as a people?". That phrasing, however, lacks the poetry and idiomatic reference (see sibling comment) of the original.