There's a difference between over-paying suppliers and being literally the best in the world at squeezing suppliers, which Apple is IMO.
You can argue (and I will argue) that Apple's skill at squeezing their suppliers while still maintaining reliable supply chains is a huge part of why they're the biggest company in the world (and also why Tim Cook and not Scott Forstall or Jony Ive succeeded Jobs). That squeezing has real human costs, which Apple is insulated from.
They've outsourced the majority of their labor (and related liability) to a global cadre of high-volume, low margin suppliers, while capturing most of the value created by that labor by owning the Apple brand and maintaining tight control of information, resulting in an asymmetric risk/reward profile for Apple inc.
I fail to understand the negative votes (above), Dell did and does exactly what is attributed to Walmart and others in this thread. It is known among manufacturers but held in secrecy like so much in that business. Source: multiple years in used computer industry in California
Successful companies overpay suppliers all the time - especially if they're growing very fast and are focused on scaling growth over cost efficiencies.
Large companies who've already scaled up their products are more incentivized to focus on cost cutting.
That isn't over-paying. That's paying what they need to get the best. Unless you have some evidence that they paid X for 5nm when they could have paid less.
No successful company is over-paying suppliers.