There is a non zero chance that Hamanu was one of Myron’s templars. Sure, he might have been a farmer’s boy when young, but the rest of his narrative is self-serving. The more likely scenario is that Hamanu was inducted into Myron’s army as a templar, found out about the existence of Rajaat, worked to undermine Myron in Rajaat’s eyes, then replaced Myron as champion. Then Hamanu rewarded Rajaat with treachery in turn.
One hint that this could be true is the strictness of how Hamanu keeps his templars in line. Hamanu know the wages of treachery.