I think Garen, deep down, has already almost certainly figured it out.
The words hung between them, and Lux looked away, remembering not to underestimate her brother. People knew him as honest and direct, with a sound grasp of tactics and war stratagems, but few ever thought of him as subtle or cunning.
That, knew Lux, was a mistake. Yes, Garen was a simple warrior, but simple didn’t mean stupid.
Garen's facade is that of being absolutely committed to everything Demacia stands for. He is an arbiter of absolute justice and has no room for anything other than white and black, good and bad. That's, at least, what he tells people.
In Taric's lore, Garen is the one to sentence him for desertion. Garen is Taric's close friend, and he gives him the "Crown Of Stone", which means he can't return to Demacia until he climbs Mt. Targon. This is widely accepted to be impossible, so this punishment is a fancy way of just saying "exile". Garen knows that Taric cannot fit in with Demacia, so kicking him out is the most merciful thing he can do. He just does it in a way that fits the Demacian tradition.
In "For Demacia", Garen accompanies Lux on an adventure, one in which she uses magic. Specifically, shooting a giant-ass-light-pillar into the sky. Then this happens:
“What happened?” gasped Varya.
“Lux,” said Garen.
Garen isn't stupid. His little sister just walked up a mountain to challenge a nightmare demon, a flashy light show occurred, and she comes back unharmed. The story ends with an exchange between Lux and Garen where she questions his sense of absolute morality, and he says this:
“We live in a world that does not allow for such nuances, Lux. Demacia is beset on all sides by terrible foes; savage tribes in the north, a rapacious empire in the east and the power of dark mages who threaten the very fabric of our realm. We deal in absolutes by necessity. Allowing doubt to cloud our judgement leaves us vulnerable. And I cannot allow us to become vulnerable.”
Garen thinks that having someone who can make the hard decisions for what he thinks is "good" is a requirement. But, as we see in Taric's lore and heavily implied in Lux's, I think this statement isn't meant to convince Lux, it's meant to convince himself. If Garen knows what Lux is, he has to come to grips with magic not being evil, and the Demacians using it as a military tool under the table. That shatters this world-view he thinks is so important, so he's probably doing all he can not to act on it.
I tend to think of Garen like I do Illaoi. Both of them are nigh-fanatically loyal to a concept. But they are humans, they have their own thoughts and emotions, and they sometimes stray from the path to obey them. Garen probably knows. He just won't admit it to himself.
