He only raced the Tour de L'Avenir once, as a first year u23 rider and also finished 8th in the u23 Peace race as a first year u23 rider (one year after he finished 10th in the junior version of the race).
Look at the top 15 of the Giro della Valle d'Aosta where he finished 3rd, that was a stacked field. The main difference between him and the top 3 in the U23 Giro was his rather poor TT.
He also "won" the Fleche du Sud (after Kvasina had some clinic stuff related problems) before turning pro.
Well yes, but it is possible to get some idea of how good a rider might be based on limited results. Arensman and Power finished on the Avenir podium as first year U23. Of course, people mature at different rates, but 48th is not much to write home about.
8th in the U23 Peace race is much better, but it doesn't exactly scream future best climber in the Dauphine. I think he was not even the youngest in the top 10.
10th as a last year junior is good, but again not more than that.
Fleche du Sud is not a mountain race so that is also not a great indication of future climbing ability.
That leaves the results in Italy. Those were probably the best, but again (my opinion only here) not outstanding.