If you were a famous person who only bought their coffee at that one place, and bought more coffee than (nearly) everyone else put together -- then yeah, I would definitely call that place "cjt's coffee place"

But the ambiguity is still there. If Gordon Ramsey loves a certain sushi restaurant and is the only one he eats at, calling it Gordon's sushi restaurant would be very confusing since it could realistically be either (or both) of the interpretations.

The same can be said for Apple, which prides itself for how vertically integrated they are

Continuing the restaurant metaphor, there is a big difference between Apple's and TSMC's relationship and a celebrity and a restaurant. The celebrity might tweak the menu a bit but it is the chef (and their cooks) which decides on how to cook it. Meanwhile, the Apple-TSMC relation is more integrated: in food analogy Apple also gets to decide what equipment they use, where to source their ingredients and shares some of their secret recipes.

Apple may be their biggest customer but at the end of the day they are still a customer. If TSMC wanted to, or had to, drop Apple as a customer they can. And they can only do this since TSMC isn't Apple's.

Does it have to be the person who buys the most?

“My company is having layoffs”. People would know that I don’t own my company.

Those aren't the same parts of speech though. The subject-object relationship is different.