It's the old Fletcher-Munson curve at work. A moderate output pickup with a lot of highs can sound louder than a mudbucker. At least up close. Well away, back in the audience, the effect can be just the opposite.
In this particular case, the aftermarket pickup wasn't just weak, it was right smack at the bridge. I just posted it because it's a vintage Epi and the new pickup he made was a sidewinder. It sounds good despite being right down by the bridge, and that's what matters.