I would guess that the number of copies printed will have been calculated from the number of published photographers with an estimated multiplier*, and they appear to believe they've done their sums too.
As a side note; I think it's hilarious that you're quoting what they say as proof that it's a fact. Like religious people quoting the bible as a warning to non-believers - just really funny

. BTW if you read it, there's no mention of their advertising or promotion strategy - they just say it's a 'commercial venture' and you read 'we're hoping for a bestseller' -
of course it's a commercial venture. No-one runs a con to lose money

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*this would be known from people running similar schemes - for instance I know of 2 local amateur poets, who were 'published' both bought a book for themselves and one bought a copy as a gift for a parent, the other one's parent bought a copy to show off to her friends. I've never seen either book for general sale - though it was for sale to anyone who wished to buy it from the publishers at the time.
The 1000 copies could be an ambitious over-estimation or it could be the lowest practical number for the economy of scale. I'd generally guess that photographers have a reasonable disposable income (due to the amount of kit they buy), so each contributor purchasing multiple copies ought to be a banker - or so the publishers appear to believe - and I'd guess they're not wrong.