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> This is an oversimplification and not always correct in all situations.
You are both saying the same thing, I guess. grandparent states that it is "possible to do flat design well" and gives examples (Apple). I don't have anyhting Apple, don't like it, but I do find their designs awe-inspiring.
To me, this comes from their investment in detail. The issues you describe "Does making "New List" a flat button and thus barely distinguishable" is real and an example of bad design. Not a proof that all flat-design always is bad. It shows that too little time and effort was poored into this piece.
I'm not sure how I would fix this particular issue. But I'm confident that within the contstraints of "flat design" and time and effort (investment) in details, this is solvable.
I'm also confident that without the constraints of flat-design, the time and effort needed would be far less. But here we are.
A desktop to-do list app that I think is super well done is Reminders on MacOS. Its basic functionality is immediately intuitive, and it gives love to power users too with its secret ability to parse natural English (e.g. "laundry at 9") for adding notification times.
This is an oversimplification and not always correct in all situations. Take the original article as an example. How did 3D buttons take away from the "more important things"? Does making "New List" a flat button and thus barely distinguishable from the "To Do" title help to enhance, somehow, the "To Do" title? Does not the strong blue in the flat button ("Add Tasks...") draw user attention to it, distracting from the rest of the UI, rather than withdrawing from the rest of the screen to allow "more important things" to shine?
If flat design really succeeded at helping to direct users' attention to what is important, it wouldn't be subject to the criticism that it confuses users. Users would be helped, not hindered, by such design, no?