The underlying premise for the claim that Machiavelli is not a "Machiavellian" is that Machiavelli did not practice the principles laid out in his most Machiavellian work, The Prince, while he held political executive office because he did not formulate his observations into theories, study the ancient leaders, or write until after his breach with the Medicis and his exile from Florence. So it can be said that he was not a Machiavellian because the principles and premises of Machiavellianism had not been formulated and articulated while he was in political power.
Another argument against Machiavelli being a Machiavellian is the extension of the first. While his principles were formulated and articulated during his exile, he never returned to political power to exercise them in actuality. Thus it can only be conjectured as to whether, at his former level of executive authority, he would have been able to apply the principles and, having been applied, he would have met with success in gaining and wielding power.