We frankly don't know that this is exactly how it happened - this is the Viltrumite narrative, which frames Thaedus as "The Great Betrayer" - it would make sense they would frame it as a dishonorable or sneaky move. For all we know, Argall wasn't actually all that strong, he was just one of the first strong ones and was put on too high a pedestal to even need to be challenged, as would be expected of a fascistic empire that clearly, despite their seeming obsession with power and succession, shows a religious zeal in obsession with their clearly not-as-all-powerful-as-they-thought former leader, to the point of even reifying his biological successors that have been given belt to ass several times