Interesting premise. My read of Churchill was that he believed the exact opposite. He certainly wrote that the issue between FDR and Stalin was less that summitry was a bad idea and more that FDR was naive and completely misjudged his ability to handle Stalin.

Likewise, I believe that Truman and Stalin did come to an agreement at Potsdam, only, in a classic case of miscommunication, each thought it was a different agreement. Stalin thought Truman was giving him a greenlight to establish a Soviet sphere of influence, and Truman thought Stalin really meant democracy (instead of “oh, yeah, ‘democracy’, wink, wink, nod, nod”). Truman straight forward and literally minded, Stalin hatching plans within plans.

So it all depends on the leaders involved. Kennedy versus Khrushchev the classic bad example, Reagan versus Gorbachev the classic good example. All this study proves is that leaders who know what they’re doing are rare. Surprise.