> their investment in teaching you their IP -- that's the theory

In your example, they cannot claim the skills and basic education that gets you up to the point of "how they mill iphones within a context" (i.e. the milling, operating the machinery, physical properties of the metal, etc)

They are not legally able to include a huge educational backbone of knowledge (that they may have taught you) as their prioptary IP. Again, NCs don't prevent that disemination. That disemination can happen without employment.