I don't think it's disputable that what Enron was doing, by the end, was fraud. 'The Smartest Guys in the Room' got a little too caught up in attacking mark-to-market, which itself isn't intrinsically fraudulent, but boy can it be misused for fraud, and the Enron guys absolutely and inarguably used M2M (among many other things) for fraud. Wilfully and knowingly.
Life is indeed shades of grey, but don't get so unmoored in your relativism that you end up giving cover to people doing genuinely bad things.
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.