I think what he means with "needs more than 1 vendor" is that there exist a competitor to Dell (or now HP) that provides roughly the same service they can switc

I think what he means with "needs more than 1 vendor" is that there exist a competitor to Dell (or now HP) that provides roughly the same service they can switch to if they are not happy with their services. With Apple the only supplier is Apple. There is no other supplier you can switch to if the relationship with Apple gets to the point that you can not work with them anymore. This ignores the operating system from the equation. Basically there is no effective replacement for Windows or macOS (or Linux). Whichever platform you pick you are stuck with due to legacy/having to retrain hundreds (if not thousands) of workers if you want to switch.