Sylvia is not a natural footballer. He has great skills and is a terrific athlete with his high leaping and his strength and power, but he has never really displayed the pure football instincts that the top players have. Things like awareness, peripheral vision in tight contests, the ability to deliver a handball in traffic to release a teammate.
I didn't see him play in the under 18s but I wouldn't be surprised if it was his brute power and strength that regularly resulted in his high possession totals, rather than genuine footy nous.
There's no doubt that recruiters would look at how a player gets his possessions at under 18s level and consider the reasons why that player is getting a lot of the ball or isn't getting a lot of the ball. This would be far more important than simple stats like how many contested possessions or clearances a particular player averaged over the year.