It's amazing that he doesn't consider himself to be a good programmer. In what other profession would someone as accomplished as Persson say that?
There aren't that many professions were a decent piece of work that makes one person happy can scale up to make a million people happy with minimal marginal effort.
Pure technical ability is a dime a dozen. Check YouTube for a zillion teenagers who can play shred guitar as well as anyone. But inspired creativity is rare and valuable, and execution requires only a certain level of technical ability, not mastery.
Quality of programming and success are not necessarily related.
And from other sources (can't verify) minecraft code is reported to be quite bad, which is the main reason many of the bugs weren't fixed very fast.
Having said that, he had a vision, executed it well and made a lot of money. My hat is off to him for that.
I dunno about you, but writing truly readable, intuitive, easy to maintain code is quite difficult. And you'll likely never get it right the first time. Refactoring for readability takes consultation from other human beings, and lots of time.
And when you're writing software for a business, it's really important to put in that time. You know what you're writing is going to last five or ten or more years and will be seen by dozens of people. It _needs_ to be good.
But for a guy working independently on a novel project? Fuck all that, I say. Just iterate as fast as you can towards the vision that you have while keeping some minimum standard of readability/maintainability. It doesn't need to be very good code, it just needs to be good enough that you can go back to it later without being completely confused.