When Stan Lee told us the Fantastic Four got their powers from “cosmic rays” he was using a real term in a completely fictional manner. Much as he later used “gamma rays” to create the Hulk, or “transistors” to power Iron Man’s armor.

Years later he used the “power cosmic” to describe the forces wielded by Galactus and his heralds, but we can be sure he did not intend that use as a reference back to the FF’s origin.

So, no, imo the FF are not empowered by the same quasimystical energies that juice up the Silver Surfer or Terrax.