I think the slam dunk against the myth is simply that there's no evidence for it at all. If you have no primary sources or archaeological evidence, and the only "source" is tour guides, then there's no reason to believe it.
It might be true or it might not, but talking about it as if it's definitely true is simply wrong.
There's no evidence for an alternative either. We simply don't have surviving documents that talk about the logic of why staircases were built that way, which is hardly surprising given that even nowadays when books are dramatically cheaper to produce, architects don't tend to write down the logic they use when designing the details of functional structures.
The combat explanation fits the evidence as well as anything else. Perhaps it's not proven to be true, but it's certainly not proven to be false.
It might be true or it might not, but talking about it as if it's definitely true is simply wrong.