Interesting, it does state that Afars enslaved some Amharas. However, Afars in the end got conquered by the xabashis and some Afars got assimilated by them and even became Christian. some of the people of Wajirat, northern Tigray, have Afar lineage. 'Irob' is a Saho clan that is fully Christian and they're a sizeable group. Xabashis and Somalis and other Muslims enslaved each other during wars but xabash highlanders were rarely made slaves.
Yes it was rare, but it happened in some cases.
Among afroasiatic speaking horners slavery mostly affected Oromos and Sidamo but they had a custom of selling their own people as slaves to punish certain crimes.
Amhara an tigrays were rary enslaved my muslims and Somalis were probably never enslaved to a significant extent.
Anyway there is a difference between being conquered and being enslaved but we can still say that in some cases habeshas were subjugated .
Oromos technically conquered the habesha during the zemene mesfint times (18th and mid 19th century) the Ethiopian monarchy was defacto destroyed and Oromo chiefs from the Yejju tribe ruled over habeshas in the highlands.
Many people who today identify as Amhara have oromo roots from that period, almost all the Ethiopian monarchs after Menelik II have some oromo ancestry.