That's true, and Paul Boag is capable of much more insightful commentary, but I suspect he was writing for an audience who is new to IA/UX/IxD. I haven't heard many of my peers perpetuating those myths for the last few years either, but I still do come across the occasional client who suggests that 'users don't scroll' or that '3 clicks or less is what we should aim for'. I would hope UX beginners these days aren't reading the same websites or design books those clients are/were, but how are they to know?
I'll put my hand up as once propagating those ideas. Looking back at those rules now I can laugh at myself for taking them seriously, but I was sincere at the time. Perhaps it was a desire to try and see patterns in user behaviour, to establish some formal guidelines, and perhaps there was some relevance in the myths once upon a time when web design was sometimes little more than plonking a few thousand words of content into a design and before touch screens were a thing.
But I appreciate how far we've come from relying on conventions and 'common knowledge' and instead following a more robust design process that doesn't make those assumptions - one that instead relies on balancing user feedback with the business intent.
Not to say we've got it all right yet either. I'll have more of a think about that, and in the meantime will "be careful what theories and rules we believe. Instead we need to research the hearsay and prove it for ourself."