Athletically and versatility wise it is hard to compare him to Thilthorpe in terms of draft outcomes; you are comparing 195cm with one that is 200cm, and whilst both are quick (both sub 2 seconds for 20m which is very fast for big men) but Thilthorpe is again even quicker that and has a better jump. In a recruiters eyes, Thilthorpe is also very versatile and can play both ruck and forward; this gets you even more draft day points as a tall prospect. Still Treacy's speed is above average for AFL.
A better comparison at the top end would be Logan McDonald as a player who is only going to play forward; Logan had an outstanding season beating men and being right up the top of the goal kickers in the WAFL. Whilst both are really good kicks on goal, McDonald gets his goals through a range of different ways, whereas Treacy as far as I can see he relies very heavily on lead-up mark and goal, which you can't rely on in AFL proper (he does seem to have the body to do other things though). McDonald also has for years been able to get a run on on goals and kick large numbers in the one game; which is an area Treacy doesn't do yet.
The other one would be Jamarra Ugle Hagan; both are the same size, speed and dedicated key forward types, but Ugle Hagan gets his goals again in many different ways (massive overhead marking, crumbing etc.) has a massive leap, and can play like both a small and a tall forward. Worse comes to worse, Ugle Hagan can probably play wing or in the backline; Treacy can't do that. Again Treacy loses out on the versatility of how he can get those goals, versatility of position and doesn't have the raw X factor the Ugle Hagan has.
If he had kicked say 6 or 7 goals for Vic Country, then I think he could have elevated himself into the conversation for a pick 30s - 60s (Shannon Neale territory where he is slightly versatile than Shannon Neale about as athletic but has more results, also has the advantage over Neale of coming from the state with the most clubs). However, I think had the U18s gone ahead, a lot of the ball movement would have been drawn to Ugle Hagan and so it would have been difficult for him to put those results on the board.
I guess everything you have jotted down points to Treacy developing - maturing super quick over a very short period of time. Calculated decision by the club.