I wouldn't say that's a general statement. A lot would depend on how the plastic film was made white. If it's a calcium carbonate (gound limestone) filler, yes, it's not highly UB absorbing. If on the other hand it's done with a decent amount of titanium dioxide, it would be a very good UV block -- TiO2 has high ultraviolet absorption.
There are plenty of pharmaceuticals packaged in white plastic -- a lot of OTC stuff, for example, but plenty of prescription ones, too. I believe the bottles dispensed by a pharmacist are amber to block UV and retain some transparency -- just like beer. It's not because amber is inherently better for UV.