Same could be said about Ciro Immobile outside of Serie A, he was so bad while playing in the Bundesliga or abroad for that matter. In Serie A he scored for fun.
So in short, your point doesn't make much sense.
Different era, last 10-15 years serie A has been shit, focus has been on attacking and shit players like immobile has flourished. Trezeguet and Batistuta were of a different generation and time. Theres no denying a player could flourish in some leagues while suck in others, weve seen it time and time again. Then theres tactical systems..
Another difference to what ive already previously mentioned between them days and now is facilities.
Nowadays theres cryotherapy, contrast water therapy (CWT) - which provides a 85% recovery boost post-match, GPS tracking, so much science and tech. Modern nutrition, sleep analysis, and load management that its helping keeping players their physicality. All this and more when used efficiently takes a player to thr next level. At Bayern they know exactly how to utilise this. Add this to a generally defensively poor league( or the vast difference between the class amongst teams), tactics, mental training amongst others and you got yourself a top player. It is the sum of all parts.