Raphinha scored twice, with Karim Adeyemi, Lamine Yamal and Gabriel Jesus also on target for Barca. Sem Steijn scored Feyenoord's only goal in their Champions League opener.
Van Bronckhorst said his side tried to play proactively but was disrupted by conceding early. He viewed the match as a lesson in sustaining pressure and also praised Yamal.
The return to Camp Nou carried personal significance for the Dutch coach, a former Barcelona player. His assessment of the hosts' strength and title prospects is a post-match opinion at the beginning of the European campaign.