I'm not sure where this misconception came from. There is no proof Barre wanted to march into Addis, his only goals were the capture of Jigjiga and Dir Dhaba aka Galbeed essentially.because Siad Barre wanted to march towards Addis Abba.
and play them off eachother and tried to get what he wanted out of them.
IIRC, you posted another source showing this was the case as well. Barre kept America at arms distance even after he went to their side in the 80s.