The flash-of-inspiration mode of creativity has been dramatized so much that most of the population assumes that's just how it's done. It also seems way easier than hard work, so people want it to be true when they try to be creative. Simultaneously, it seems way less attainable than plain old hard work (the requirements seem to be mostly circumstance), so people also want it to be true when they are not trying to be creative ("My great idea just hasn't come to me yet!").
JC is reminding us of the hard truth: the reality behind the stories is that creativity is the prize of determination, not serendipity.
If that's not the 'apple tree' of software development, I don't know what is.
I'd bet my life that if you have no time to sit and think, just a busy-body answering phone calls, emails, you're not coming up ideas. At one point JC did sit and think, hard. Then he implemented hard.
Through the whole process he had isolation, so he really was 'under the apple tree'. Irregardless if his process involved building prototypes of his various ideas.
He in my view, failed to see that he just filled his 'apple tree' time with tinkering. Some people plant a garden while thinking about code, or focus intently. I'm not sure there's much difference between methods, it depends on the field and person.
The most complex fields, involving what I'd consider true genius cannot be tested. It's just too general of a statement, and only relevant to him or a portion of the population.
But this is Carmack as I've always seen him. Technically brilliant, missing the big picture (his games were in some cases restricted simply to show off a technical feature eg. the Doom 3 flashlight), and fairly narrow minded.
Inspiration can sometimes show you which path to take, but in the absence of inspiration, simply brute forcing will keep you moving forward as well.