University types still harbor love for Lenin privately so it doesn't surprise they would love Zinn. As for Deutscher- that question is murky. Trotsky was a great polemicist and political speaker. But he lacked the patience for the daily grind of pratical politics that Stalin had mastered. But supposing Stalin dropped dead of a "heart attack" in 1926 and Trotsky assumed the reigns of power (again unlikely he would have held them for long but let's suspend that for the sake of argument). Trotsky was even further to the "left" than Stalin. I actually think Trotsky would have been even more of a blood thirsty killer of peasants than Stalin (but easier on the rank and file party members). A Trotsky run Soviet Union would have been even more of a disaster than a Stalinist one and with a bigger body count. Deutscher might have seen this and recognized it when writing his history. Deutscher was critical of Stalin from the perspective of an "if only" fantasy. If only Trotsky had run things it would have worked out great was his thinking and it runs throughout his writing. It is a fantasy but it did lead him to write some good stuff. Would he have held onto to this fantasy if Trotsky ran things in Russia and it failed even worse? Most likely not - but he would have found another fantasy excuse on the left on which to hang his critiques from.
I hope that rather long paragraph makes some sense.