Resilient flooring (WPC, SPC or LVT), in general, is waterproof because water doesn't impact the integrity of the product itself. Meaning that a resilient flooring plank can be submerged in water or have standing water on top of it for an extended period of time without causing damage to the product.
However, having high levels of moisture underneath the flooring occurs with an uncured concrete slab or when another continuous source of subfloor moisture goes untreated, it creates a much different situation that produces effects that create high rates of hydrostatic pressure (pressure exerted by a fluid at rest due to the force of gravity). This high pressure will rise and escape and will do so at the least point of resistance, which is at the seams of installed flooring.
The effects of this high pressure escaping will cause cupping, raised corners or peaking at the seams. Using a poly film will help mitigate the moisture from escaping through the seams and will force the moisture to escape around the perimeter of the flooring, at the walls.