Yeah, I think toward the end there you're getting closer to how MLS hype actually happens.
As for the substantive argument, Rossi's stats at Peñarol were good but then again Peñarol is practically the Man City of Uruguay: their goal differential last year was double the next closest team's. Libertad is a good team too but not nearly as prolific, so you'd expect their attackers to be involved in fewer goals. Plus then you've got to know what Rossi and Medina's tactical roles were, who they were playing alongside, what kind of game situations they earned their minutes in. You've got to know if they were taking set pieces or penalties. You'd also want to compare their performances in Libertadores, where Medina scored and Rossi didn't (meaningless, I know, but that's the point).
In the end, trying to suss out a player's quality from stats like these is hopeless. You need a lot more context than we can scrape together from the internet, so you've got to trust the scouts did their job and see what happens when we've watched the guys play for a season or two.