The Sens preferred to pick a natural goal scorer with a track record in men's league as a 17 year old. They probably saw what's apparent - a still growing kid with good size, skating, and shooting abilities. The latter is supposed to be high-end, but he is missing in action in this tournament.
Wrt to JJP, it's possible they found him to be redundant in terms of young playmaking wingers already in the system, and went to draft a potential goal scorer who needs a lot of development time to get there physically and mentally. The latter specially apparent in this tournament.
The Finnish coaching staff put him on the first line to start the tournament based on his performance in Liiga. Jarventie was held pointless in Liiga in his last 3 games played IIRC and came in cold to start the tournament. The only thing he is warming nowadays is the bench.
But I am not worried. We still have a shooting LW prospect with speed and size. But one that requires significant coaching and training on how to play, in both zones, when you don't have the puck on your stick.