People also commonly pronounce "nuclear" as "nukular" and refer to spiders and scorpions as insects or dolphins as fish. That doesn't make it correct. It's one of those things that's too much trouble to correct every time it comes up in casual conversation, but when we're having a discussion specifically about dubs and subs, I hardly think it's out of line to ask people to be a little bit more exact in their word choice.
There's a difference between saying, "Let's not watch the dub today, let's watch the original," and saying, "We should always watch the sub because it's the original and the dub is not." In the former, the incorrect assumption that the sub is "the original" is incidental: it's a poor word choice, not an error in judgment. In the latter, it's central to the point being made and deserves to be corrected.