They are not "THHN" they are just a 90c rated conductor of some kind, since that is all the listing commits to.
As a practical thing we might assume they are THHN, just because of manufacturing expediency but we can't prove it.
For that matter you might even assume they are THHN/THWN but you know what assume means.
Who would have thought the covers on NM conduit bodies and the straps for RNC were not made out of the same plastic as the raceway but I had a Carlon rep tell me they don't have to be (in the listing) and in fact, at the time they were not UV protected. I don't know if they ever fixed that.
