Just because the flotsam isn't visible doesn't mean it doesn't exist. The concentrations of the garbage are quite high. And being microscopic, they're very hard to clean up.
They don't say it doesn't exist, period.
They say it doesn't exist in the particular (mythical) form described.
If the Lock Bigfoot turned out to be a stray gorilla from some local zoo, he would exist, but the "Bigfoot" as described would still be a myth.
(Myth after all is commonly based on some historical basis).