Sorry if I've got the wrong end of the stick here, but I think maybe the original question was more about what we think defines a mountain bike rather than necessarily pulling apart the quote. It's a tricky one really - it feels like a similar question to: 'what's the difference between a pony and a small horse?' If you show me either I can probably tell you which is which but I would struggle to define why on paper.
As someone else has pointed out - when you look at what road bikes can be put through (particularly over the cobbles in Belgium) you'd have to say that they're probably tougher than a lot of our beloved MTBs from BITD. And when you think about what cyclocross bikes get up to - really it's not that different to what I used to do on XC races back in the 90's. Neither of them look like a 'mountain bike' to me though. So it's not about solidity or capability, necessarily.
In theory most bikes could 'get up a mountain'. The klunkers got up to the top by van, from the pictures I've seen and from what I've read (happy to be corrected on that!) - so by the definition of, 'capable of being got to the top of a mountain by hook or by crook' pretty much any bike fits the generic term; assuming you want it to.
I think what this has got me thinking now though (which is a bugger because I should be concentrating on bug fixing today!) is: what makes my Zaskar a mountain bike and my fold up shopper just a normal bike. I guess the term, 'Mountain Bike' itself /could/ be applied to a specific set of criteria (which might be what the OP and original quote is hinting at) vs any bike we ride off road and call a kind of mountain bike.
I think I need to let this pound around my head for a while
So I'm going to shut up now, before I get fired, and let my thoughts congeal for a while!
Edit: Just thinking about it I think the Klunkers in a van was more the Repack race than the general case (before I get bombarded!
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