I don't think it really made that much of a difference. It didn't hurt him, but a year in Liiga or KHL would have probably done the same. The thing that has helped Rantanen probably the most is his off-season training. His trainer said already two years ago that he will have a break out year because he's such a monster physically.
I have seen some people mention Koivu and Granlund as someone who benefited from AHL. That's not really true. They played there pretty much only because they happened to come over during the lockout seasons. Both were good enough to play in the NHL from the get go. Even if Granlund was pretty slow and overall raw physically and needed to work hard during the off-seasons to get to where he is now.
And for every Rantanen we have 4 guys like Pulkkinen, Rajala, Pesonen and Filppula who dominate in the AHL and yet never amount to anything in the NHL. Rantanen is the only Finnish forward to have a successful season in both AHL and NHL. Even Puljujärvi has played over 50 games in the AHL and still looks completely lost when he plays for the Oilers.