My read of the book was that at the beginning Enron was attempting to use M2M for their "Gas Bank" concept, and at that point it wasn't obvious that it was wrong, and it wasn't fraud either. We now don't accept M2M accounting for what they were using it for, but they were seemingly the first (or first to get noticed?) to use it in the way they did and if things worked out differently maybe it would have stuck. In this way I think it's a bit of a case of "history is written by the victor".

By the end they were doing clear and obvious fraud, particularly in how they orchestrated the incoming funding for projects, and it had become clear that M2M was not working, but I don't think this was the only possible outcome.