I wonder if these jobs are primarily for certain contracts which either require US made equipment or give a bonus/subsidy for it. Are there some defense contracts which they can get by snapping a Chinese made mother board into a Chinese made case here in the US rather than in China?
The details are limited, but the reports I have read suggests that there will be a fairly extensive assembly and manufacturing process in the US. Many iMacs require careful machining of case parts. Also, motherboard assembly lends itself to US manufacturing already because it is nearly lights-out automation.
I think this is a substantial manufacturing investment, not a Potemkin show.