Those fabrics are made of plastic, which is derived from oil, which forms over long periods of time from buried decaying plant and animal matter. 70% of the Earth's oil is from the Mesozoic Era, which encompassed the Triassic, Jurassic, and Cretaceous Periods, so formed during a time when dinosaurs lived and jokes about oil being made of dinosaurs are common. Oil is actually made of plankton rather than dinosaurs, though.
Even if it wasn’t, it’s a valid point to argue that found roadkill is vegan (but still not safe to eat, RFK Jr), because no one is exploiting anyone else. There are lots of reasons for vegans not to embrace petroleum products, but the well-being of the animals and plants that formed the petroleum isn’t one of them.