True it was a PR exercise for Trump, but Foxconn had been talking to Wisconsin long before Trump shuffled along and claimed credit.
Foxconn was probably talking to a lot of governors fishing for the biggest subsidies they could find, and making promises they knew they wouldn't keep about the size of the plant. If the subsidies are large enough it will make sense for them at some smaller size.
Foxconn has to keep moving the goalposts to keep the subsidies from being pulled entirely, but I don't see how they can ever claim many of them unless they can prove they've actually brought jobs to the area. Nevermind that with the proposed location on the Illinois border it wouldn't even exclusively benefit Wisconsin so the subsidies would come out higher per state taxpayer than claimed.