For a start, there's currently no legal framework that could force Apple (or any other American company) to move the bulk of its manufacturing to the US. There's also a question of whether or not any build-it-here legislation would be effective without going too far in punishing companies that insist on offshoring. To boot, any successful moves would require a balancing act between US interests and preventing chaos in foreign economies. There wouldn't be much point to making more devices at home if you have fewer customers, after all.
On top of this, the claim ignores the fundamental problems with domesticating production. It's not so much the cost that would hurt American firms as the supply costs and the skills gap. Chinese manufacturing giants like Foxconn can bring in both resources and thousands of trained assembly workers at the drop of a hat, but that's much harder in the US -- most resources are abroad, and workers are frequently over-qualified (if they're qualified at all). Apple can afford to make the Mac Pro in the US because it's a small-volume product built using highly roboticized techniques, and it indirectly fosters roughly 361,000 jobs through third-party suppliers. It'd be another matter entirely to produce mainstream products like the iPhone or MacBook, especially if more humans have to get involved. It might not be feasible to make high-volume devices in the US, even if Trump gets exactly what he wants.
As such, the promise rings hollow. It's a well-meaning idea (more jobs! Stronger economy!), but there's no viable plan attached to it. Unless he can trigger a wholesale shift in the American education system and enact sweeping legislation while he's at it, it's likely that Apple and its rivals will be outsourcing most of their work for years to come.
[Image credit: Darron Birgenheier, Flickr]