My wild guess is that Cook cut a deal with the IRS so that they build in the US, but get tax benefits other companies don't get, so that it looks good on the administration - like the tariffs are working - and still benefits Apple.

I don't think Apple wouldn't find a cheaper place to manufacture Macs than the US. The US is literally the most expensive place to build.

That, or the Mac Minis are 100% asembled by robots, which is also a possibility.