He meant that the 'people that have heard this' have no idea. I'm not sure whether his sentence can be properly interpreted as such, but even if it's grammatica

He meant that the 'people that have heard this' have no idea. I'm not sure whether his sentence can be properly interpreted as such, but even if it's grammatically dubious, the intention is clear (to me). He probably knows that the effect is vastly overstated in popular accounts, as other commenters have pointed out.