I think the documentation for medieval military architecture choices is extremely sparse. Most of the writers were monks, they weren't so interested in military matters.
The author is making an "absence of evidence is evidence of absence" error.
The author is making an "absence of evidence is evidence of absence" error.
He isn't making that kind of error. The burden of proof is on the people making the claim. Otherwise any sort of weakly plausible reason for choosing the staircase direction would be true because there is "only" absence of evidence.
Absence of evidence here means that while you can't rule it out, you can't positively state it either.
Right, so calling it a "myth", as in the headline, is incorrect. That would imply that you know it to be false. The author is taking the lack of evidence to mean that it can't be true.
And as for the burden of proof, as with any hypothesis, anyone is welcome to try to falsify it. I just don't believe that the evidence here has done that.
We think of monks as the only literate people in the past, but that was far from true. The common person might not have known how to read (I've seen good arguments they did a little, but I'm not qualified to evaluate them) but the rich did, and they hired scholars who could read to study things that were important to them. Any rich person who didn't have scholars studying military matters wouldn't last long in war as many seemingly obvious ideas don't work in real battles.
Quite the opposite, the theory he's questioning has no evidence, merely a good story justifying it. In the absence of actual evidence of the motives behind these designs, you should be agnostic, which is what the author is doing.