Garen had always been very clear about his own strengths and weaknesses.
Although he felt a hint of danger from roxia, it did not mean that roxia was an existence that garen would fear.
In other words, anyone whose speed was within his reaction range, and who needed to fight in close combat to pose a threat to him, was not considered a real enemy by garen.
There was no other reason other than the fact that he could do whatever he wanted with time freeze.
What garen was truly afraid of was the kind that was far away from him, and a high-level spell could instantly enter his soul.
Once a spell was successfully cast, its attack speed would be faster than garen's extreme acceleration state. Some spells could even be used on the enemy before they realized it.
However, roxia was obviously not this type.