I wonder why he wasn’t playing at a more competitive level
I recommend you watch his interview. It will answer a lot of your questions:
Long story short, he was drafted in the 9th round of the 2022 OHL draft, but he decided to go to prep school instead to get more playing time, better development, and also a better education.
Here's also an article that has details on it:
Letourneau making sizable impression heading into 2024 NHL Draft | NHL.com
"St. Andrew's has a great track record of guys getting drafted into the NHL and playing in the NHL," the 18-year-old center said. "So I didn't really think that was going to be an issue there."
Letourneau was selected by Owen Sound of the Ontario Hockey League in the ninth round (No. 174) of the 2022 OHL draft, but opted for prep school.
"I had always wanted to play in the OHL growing up," he said. "I had wanted to play as a 16-year-old. But when I went to St. Andrew's and saw a lot of guys that played [Division I hockey] and what their careers looked like, I gave it up pretty quick."