Orcas are in the dolphin family which is a branch of the whale family, specifically those with teeth rather than baleen. Compare how humans are in the ape family which is a branch of the primate family, specifically those that are less arboreal and lack tails. If we can say humans are primates, we can definitely say that orcas are whales.
Wait until you learn the original joke dialogue was “Are you worried about mad cow disease?” and “No, I’m a tractor.”. I kind of messed up on the first line previously because either work with the punchline.