I was reading something about this recently, but unfortunately I can’t remember exactly where. So I’ll take a stab at remembering, and hope it isn’t too inaccurate.
I think that they were meaning that’s how long the period of measurements is before the brain classifies an event as “now”.
It would be nice to say that the “now” you’re experiencing is actually the last 8 seconds of what your senses have measured, but that’s not quite right. It’s a combination of what you have measured in the last couple of seconds, and also what comes in the few seconds after now – this is sort of integrated back into your memory at a later point. Hence where deja-vu comes from.
So it’s not the absolute, physical definition of “now” – it’s more the psychology of “now”, how your brain deals with an event happening “now”.