Not really the system, it's the trait's he gave the character. He needed to reach the proper stamina level to unlock it. The trait gives him the ability to use the swordsmanship the way he wants. He's played the game 27 times, so knowing how to fight from seeing it on the screen you tend to have an idea of how it's done and are able to put it to use when you have the skill to put it to use. The system only allows him to see his growth and reward him for completing quests and such. It doesn't just hand out things because he advanced a point or two.
You don't even know if the body had trained before his possession or not, he might have but we don't know the backstory further. Stamina is very important to do anything really and without knowing what happened prior to his possession we can't say the system helped him in that month he was doing the stamina training. Plus the girl had said she'd seen him swing the sword a few times as well as we saw when he got to the training hall the day he possessed the body he swung it 10 times. He might have gotten a feel of some idea on if the body has some idea of swordsmanship or not at that time and might have practiced some swings after stamina training. We're not sure on what transpired over the month besides his stamina training since they time skipped it after a day or two.
i'm only on chapter 8 but it seem like everything was kinda handed to him, even the red hair girl feeling for him was too quick and it didn't make any sense, yes he did "train" but that was just running around for a few day and suddenly he's able to defeat 2 person who has been training since probably birth, ig we could say the system help him but it just didn't seem like he earn anything yet.