M4 and M5 are literally general purpose computing parts. Apple literally owns the most profitable general purpose computing platform with the iPhone.
The iPhone is anything but a general purpose computing platform. Apple actively prevents many purposes.
Perhaps this was worded poorly, but the parent is referring to inability to source these processors from Apple and use them in other (non-Apple devices).
As in, they don't sell you the parts, they only sell you the entire product. If you don't want the entire package, the processors alone are irrelevant.