I wouldn't be at all surprised at the decoder chips for the "extra" RAM being damaged or dummy samples etc... along with the chip itself! An 8KB jobbie wouldn't have been very expensive at the time, vs the import tax (especially as the entire 64K machine only retailed for about twice the tax itself), but a dead one would have been very cheap indeed.
And, after all, other companies such as the rather closely related Sinclair had a habit of using a brace of damaged higher-capacity chips in order to create a cheap computer with only about half the memory that the chip labels and complement would suggest... Even Intel sells their high end CPUs that have certain non-functional parts (e.g. 386s with damaged address/data lines, 486s with damaged FPUs, PIIs/IIIs/4s with partially or wholly damaged caches/cache controllers) as significantly cheaper budget models...
One expects the manufacturer of the 4164 et al found themselves a minor additional revenue stream by selling partially or wholly non-functional examples for "nonstandard purposes"!
However, I'm a bit suspicious of that excerpt from Sir Alan's autobio. Maybe it is actually taken verbatim, but I'll bet he wasn't actually the author of that. Could be he mentioned it in passing and his ghostwriter did a bit of research and fleshed out the details. In most of his appearances on TV, radio, in print etc he hasn't presented as a massively technical person - he's a minor genius when it comes to business matters, and manages to sell people what they actually want at a cunningly low price*, but doesn't get hugely bogged down in learning the techie details if there's someone on his team who can look after that whilst he deals with the human and financial factors. It's far more likely it actually went along the lines of:
Amstrad Lackey: "Mr Sugar, we've a bit of a problem with our flagship computer in the Spanish market..."
Sir Alan: "What is it?"
AL: "The Spanish government want to promote a home-grown computer somewhat similar to it, so they're putting a sixty quid tax on everything with 64K of memory or less... And ours is a 64." SA: "Bloody hell! We only sell them for a hundred and twenty! Hmm ... how much does each... 'K' of memory cost?" AL: "Oh, probably about 50p or so, I guess? There's a lot of other parts in the machine after all." SA: "Well, then - get the boffins at the factory to put 65K in it instead! And put the price up to one-thirty... no, one-thirty-nine. May as well make this work to our advantage. Go on, make it happen. Right, what's the next disaster to befall us today?"
Also, sorry I can't find the 472's actual sale price, but I would expect it's slightly more than the 464 was. After all, if all your competitors are suddenly having to sell much more expensively or have been effectively priced out of the market altogether, whilst you've found a handy loophole, you can afford to raise the sale price a little. So long as it's still below whatever the home-grown Spanish competitor is.
(* or at least, most of what they want, and the idea of the rest of it; after the CD-door-closed detector switch on my Micro 1000 hi-fi stopped working after a bit too much physical abuse, I spent ages trying to find another system that did all the actually useful things it could (e.g. Aux input, an inspired addition) with a similarly low price and a minimum of useless fussy extras... as well as the ability to =SEARCH= back and forth thru a song on the CD player instead of just skipping it! (Turns out it's actually a function you don't use very often at all, but do want SOMETIMES... but presumably is quite complicated to provide for, electronically. So they dropped that, and instead gave the thing a comparitively high-quality display vs other cheap CD players) For at least a couple of years I instead struggled on using it with a bodged repair using a matchstick, a paperweight and some blu-tack before I found an outwardly dowdy but internally excellent Grundig that I could afford... though it still didn't have as many radio presets (only 6, where the Amstrad's 10-per-band were just about perfect for my local area). Both of them still lurk at the back of the cupboard in my flat needing various minor repairs, not quite broken enough for my heart to stand throwing them out...)