How is the polyethylene being attacked?
Polyethylene is attacked by microbes that contain oxygenase or oxidase enzymes. These enzymes can attack C-H bonds even at moderate temperatures.
The only agents that I have heard of that can degrade a fully-saturated hydrocarbon at room temperature are fluorine (and related nasties), other halogens (with UV light), magic acid (which is extremely reactive with almost anything).
One prominent example of a hydrocarbon degrading enzyme is methane monooxygenase, which is capable of catalyzing methane oxidation to methanol even at temperatures as low as 0 °C. The C-H bonds of polyethylene are about as reactive as methane's. (Actually, they're slightly more reactive.) Thus enzymes can do the job. They don't need elemental fluorine or magic acid.
However, your point about halogens reacting with hydrocarbons is very interesting. Several research groups have reported that adding halide salts to aqueous (and aerobic) suspensions of bacteria accelerates the rate of oxidation.
It's possible that this finding can be explained by the involvement of haloperoxidase enzymes in the degradation. However, the mechanism of microbial polyethylene degradation is a very new, and very active, area of ongoing research.