Dispersing a coherent stable wave of photons capable of piercing anything it touches is not the same as dispersing a hard light structure. Hard light is something you can interact with, like a physical structure and you only need to destabilize it using enough energy to interfere with the force keeping it together, it's similar to shattering glass. But a laser beam is non physical, like light emitted from a flashlight, but it's so concentrated and dense that it produces enough energy to destroy objects through just vaporization or burning through them. It's a mixture of fiction and realism, because "hard light" doesn't exist, we can make holograms but that's not the same thing. But the reason Franky was so shocked, an actual scientist, is because interacting physically with laser beams should be impossible since they are non-physical, they have no mass or resistance, hence why Kizaru mentions physics.