I think that's part of the problem, Varoufakis is intelligent and an economist, yes, but really not a diplomat, and his job here as Greece's main negotiator was more a diplomat's than an economist's job. He was very naive, as his book somewhat admits, about how EU politics work (and to some extent about Greek politics as well, where he assumed Tsipras was on the same page as he was).

Stathis Kouvelakis, a member of Syriza's Left Platform faction that later split from the party, had this criticism [1] about both Varoufakis and his successor Tsakalotos (another economics professor), which I think has some truth to it:

Tsakalotos said he was very disappointed by the low level of the discussion. In the interview to the New Statesman, Varoufakis says very similar things about his own experience, although his style is clearly more confrontational than Tsakalotos's. From this it is quite clear that these people were expecting the confrontation with the EU to happen along the lines of an academic conference when you go with a nice paper and you expect a kind of nice counter-paper to be presented. I think this is telling about what the Left is about today. The Left is filled with lots of people who are well-meaning, but who are totally impotent on the field of real politics.

[1] https://www.jacobinmag.com/2015/07/tsipras-varoufakis-kouvel...