I see you’ve been reading your science textbooks (or browsing Reddit ;).
While berberine is a potent activator of AMPK, which antagonizes mTOR (the main enzyme that regulates muscle protein synthesis), that doesn’t mean it's always going to inhibit muscle growth. After all, muscle contractions themselves activate[1] AMPK, and no one believes that working out slows down or prevents muscle growth.
1 http://www.ncbi.nlm.nih.gov/pubmed/24624087