Impro is a handbook for running a cult

Cult may mean many things. In its pejorative form, it often means a group that engages in exploitation of its members usually for the exclusive benefit of a single living member/leader.

That said, many ideas set the stage for unquestioning group think and personal adulation needed for an exploitative cult to emerge.

Given that these groups exist all over and seem to arise spontaneously in similar form I'd say its more a case of the leaders bent to exploit than the lack of a book of methods that is critical.

A pyromaniac and fireman read a book on fire science for very different ends. The book isn't the problem.