My understanding is that the last application code Gates was involved in was the TRS-80 Model 100 [1] circa 1982; I taught my kids assembly on the Model 100 and think it's great.
Allen and gates where writing code when they were in highschool when no one was writing code. They also also got 1590 and 1600 on theirs sats when it used to be super hard. These are super smart and motivated people. You don't they could master leetcode after a few weeks of practice?
[1] https://en.wikipedia.org/wiki/TRS-80_Model_100#ROM_firmware