Abiy doesn't care about Ethnicity. He was born in one of the most Ethiopian-ized parts of Oromia to an Orthodox mom and Muslim Dad, and converted to Protestantism. He was one of a small number of Oromos to join the TPLF early on when the majority backed the OLF or Derg. He learned Oromo as a baby, Amhara as a child, and Tigrayan as a teenager.
Abiy has killed as many Oromos as the TPLF ever did. He's putting them in positions in power for his benefit, not theirs. When he sides with the Afar over the Somalis, or Sidama over Wolaita, it's a tactical decision, nothing more.
He's a post-ethnic Ethiopian, which is fucking crazy, but very based too in a way.
He's an pan Ethiopianist and he may very well say to Oromos "don't break up Ethiopia, expand Ethiopia's borders instead by annexing Somalia" for example