You may be right.
When it was first on the market there was only one variety and it would deteriorate when exposed to UV - same issue as was typical with epoxy coatings.
Since then they now sell a multi-step system for rust preventative applications. Looking at their current website I can't tell you which is their original product. My guess is its the Rust Preventative Coating rather than the top coating.
According to the post here by a 'former employee' the POR-15 of the 1970s/80s was a moisture curing urethane. Much of that post aligns with information that is true. For example the statement about aluminum pigments aligns with the USDA's Forest Product Laboratories published tests of moisture permeability of finish compostions (for use on wood structures).
Interesting topic. Later I'll go check on some architectural metal that got the full treatment over 10 years ago. I'm now curious how its held up (it was not my job, but I was working on another project on the same building).