There are comments about this in the article. From what I understand the stormtroopers during the Clone Wars was the clone of Jango Fett but after that task was done and the Jedi was eliminated the Imperial forces recruited volunteers across the Empire. For what reason I am not sure if say the Empire did not want to be dependent on a single source for manpower or some theory about making copies and copies resulted in deterioration of the clones.
This guy is probably a major character wearing a stormtrooper uniform. Like Luke and Han Solo wearing those uniforms on the Death Star to sneak around. Some people seem to make much to do about nothing
It’s because about 12 years BBY (Before the Battle of Yavin, or Episode IV), the planet of Kamino (source of the clone army) rebelled against the empire. The clones loyal to the empire destroyed the cloning vats in the attack. Everyone else was killed.
If it wasn’t in any of the movies, it didn’t happen, IMO. Referring to books written years after the original films were made to embelish the overall story isn’t kosher.