Typically true, but not necessarily. I saw one of those emerging technology videos that presented a vegan leather that was made out of mycelium - the thread-like stuff mushrooms and other fungi spread underground or throughout decomposing food.

Apparently that stuff looks and feels a whole lot like leather itself once processed for this purpose.

I think in this case though, it literally is plastic. Remember when the word 'pleather' was the in term for plastic "leather" ?