All you need is an ND filter (which is basically photographic equivalent of non-polarized sunglasses) that allows you to shoot long-exposures during the day without overexposing the sensor.

It looks unreal because just like any other long-exposure photo, you can't see such effect with the naked eye, and additionally, people are mostly only used to long-exposure photos taken after dark (where you don't need a darkening filter because there's not enough light already).

So while you can sort of argue that the photo is fake (but that's a very slippery slope because no photo perfectly captures reality, not to mention neither do our eyes and brains), it is absolutely possible to shoot this (both on film and digital cameras) without any post-processing.