keystring I think the idea was that if you did not have incremental teaching that built one thing on top of the other, you may have holes in unexpected areas.
I think having holes is inevitable, even with a good teacher conscientiously making sure you're getting skills that build on previous skills. So things you just can't play until you encounter them in context (i.e., in a piece, not in an isolated exercise).
And that's ok. You encounter it in the piece, and then you learn it and it gets added to your tool box.
If you run into such holes while attempting a piece, you might back off, address that missing (the hole) and then return to the piece.
That's basically what I'm doing.... and I guess that's what I've been doing with my playing off and on for year. I find a piece I want to play, I practice it, realize it's too hard for me and set it aside and work on something else. Over the years, I've come back to many pieces and found them playable when they were not the first time I attempted them.
OTOH, I still have a long list of pieces that were too hard, and probably still all.
It's an iterative process. But getting to play wonderful music makes it more than bearable!