I generally agree with the sentiment, but it seems like the author never really shed the dependence on having an arbitrary material goal.

I think a more Zen-like direction is to just apply the "friend in a pub" test - if you can describe your motivations and actions to an (intelligent) peer and have them nod in approval, then that's it. And yes, there are many such choices, which is the whole point - it doesn't really matter beyond that threshold!

I remember that moment of epiphany for myself, when I realized that nobody else really gives a shit what specifically I do, as long as it's a reasonably fine decision.

And no, that does not mean slacking/etc. Wasting one's potential is definitely not reasonable.