That's a trivial detail of the parent's anecdote which may or may not have been exaggerated for narrative purposes and completely tangential to the point they were making. Given the subject matter it's pointless and a frankly bit insensitive to nitpick that IMO.

I am really interested in measuring working memory and how it relates to IQ, hence the details.

I admit to not taking emotions into consideration here.

If you actually were interested, you would have asked for details rather than simply attacking the anecdote as implausible.

That's not quite right. Remember that grouped numbers count as a singular number within some limits. For example, if I was trying to remember a phone number in my working set: 1-2-3 4-5-6 7-8-9-0 that would be too large, but if I group the numbers: 12-3 45-6 78-90 then now it fits. Of course, most adults will remember that number like this: 123 456 78-90

When you're multiplying large numbers you only need around 5 or 6 items.

The former- it's something of a party trick for him. Most people can chunk up to 3 digit numbers, so you really only need to remember 4 numbers (original two, result, and temp). When I was little people would try to beat him on a calculator, and rarely won.

His talent was mainly that he could factor super quickly, and had the times tables memorized up to some absurd level. So that just cut a lot of thinking out of the task.

Replying to you and vorpalhex at the same time. I can see memorizing multiplication tables for 2-digit numbers * 1 digit numbers, (0-99 * 0-9).

I am unsure about chunking for random large numbers though. Mnemonic champions chunk very large numbers (I don't have sources on hand), but normal people generally don't.

Not at all. I just did that one in my head while driving, though I'm not fast.

The original multiplicands are honestly the hardest to remember. Don't materialize all of the intermediate results, and you can stay within working memory. You'll need to chunk a little, but mostly only 2 digit numbers.

Personally, I use fingers to keep track of the 10s place, and work from left to right. Less overwriting that way, and you start with an underapproximation that you refine gradually, which means you can stop early when estimating.