" To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of particles that have a size of 0.3 µm."
That means bacteria (1-10 µm) and spores (~1 µm) but not virusses (~ 100 nm +/- a lot) and certainly not molecules emitted by a skunk corpse.
Its activated charcoal filter has the primary layer, a secondary layer that filters sulfur gases, and a tertiary layer that filters alkaline gases.
Skunk spray is made primarily of thiols and their acetate derivatives. Thiols (the main ugly smelling ones are both thiols) are organosulfur compounds, and would be filtered by the secondary chemical filter.
The gases that make the decaying corpse smell are acidic (AFAIK), so would be filtered by the tertiary layer.
Recirculating cabin air is plenty for skunk. Even economy American cars -- that no-one would confuse with exceptional engineering -- do just fine keeping the stink out.
My old GMC Jimmy had no recirculate setting. Incredible. We live off gravel roads. When following another car, I had to just stop and wait a minute for the air to clear, else I'd be choked by lime dust in seconds.
That means bacteria (1-10 µm) and spores (~1 µm) but not virusses (~ 100 nm +/- a lot) and certainly not molecules emitted by a skunk corpse.