I just finished some digging on that hypothesis:
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Went into Alibaba.com (which is the nearest thing I know in China for buying parts in volume like flashlight manufacturers do, and not from necessarily authorized/official resellers), and searched for the exact same parts I had previously searched on Mouser.com;
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Winnowing out the obvious scams, fakes and buyer traps (photos that didn’t match anything like the parts in question, parts selling for $0.01 quantity-one, etc) these were the cheapest I could find: PIC16F1824-E/JQ for $0.50 ea. in >1000 quantities, and ATTINY1616-MNR for $0.32 ea. in >500 quantities.
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As a side note, there were many more offers for that AT1616 than the PIC – so not only price but also available supply, both favors the AT1616.
So it seems that even in China’s “greyer” markets, the AT1616 remains significantly cheaper than an ‘equivalent’ PIC (scare quotes on `equivalent’ because OFC the AT1616 is a vastly superior MCU).
So, unless there’s a significantly different market somewhere over there in China (dark alley deals in Shenzen’s danger districts, anyone? 
) it seems the above hypothesis rests falsified, and that price is not the reason flashlight manufacturers use PICs instead of ATTinys.