It's more complicated. Olive oil is high in antioxidants. A recent study found that no realistic cooking process would significantly oxidize the pufas in evoo because the antioxidants are pretty good at what they do.
The processing step that the fake oils use will degrade the antioxidant capacity that you'd get from real, cold-pressed oil.
So, in summary:
PUFA's go rancid easily in absence of antioxidants.
Pure EVOO, though PUFA, has antioxidants that protect it.
If you buy "EVOO" in the US, it has possibly undergone processing (despite the label!), which means that it will have lower antioxidative capacity
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