I'll leave the search for 10% 3-month yield to the parent, but note that the math still works for much smaller yields. Rolling e.g. 12-month US treasuries will yield around .1%, which is roughly $40m annually on the hypothetical $10B. More or less risk free and with excellent margins. This is meaningful even to a company as large as Apple.
No doubt this behavior began before the Fed started their zero interest rate policy. Come to think of it, Apple was in business at the end of the Carter Administration when interest rates hit and exceeded 20% (!!!)....