One thing I don't see very often but is intensely useful is that you don't always have to make a choice. The "Add Playlist" example is a case of "why not both?" -- have it at the bottom of the list, have it in the header of the list, sure, throw one in the menu as well. The user journeys can be either "I came here to add a playlist", "Hey, here are playlists, I should make one", and "Oh, they don't have the playlist I want, I'll make one", and all of those end up in different places. The Material-style "+" in the corner is in my mind almost always the worst case -- if everyone did it, then it wouldn't be so bad, but it's a thing you never think to look for with an unfamiliar UI.
I like the discussion of "Poisson" options; thinking about the distribution of inputs is an important factor. For location/country dropdowns, I find it frustrating that often my country, the "United States of America" is put at the top of the dropdown for convenience, but if you miss that and start to scroll down, it does not appear in the "U" section of the dropdown. Just put it in both!
The "Add Playlist" example is a case of "why not both?"
One potential answer is that while two possible actions may be obviously equivalent to the designer, that is not necessarily obvious to the user. It's a classic usability problem: the designer tries to be helpful but in reality there is now a confusing variety of similar-looking ways forward, and then the user gets analysis paralysis and gives up. The mistake is often compounded by having equivalent things look slightly different, for example using different types of controls, or using slightly different terminology or phrasing.
The more buttons, the less visibility per button. One (clear) button in the right place beats two separate buttons, when other buttons are taken into account.
I think that's making assumptions about user attention that aren't valid. The user is not looking for all ways to do something; they are looking for the first way that satisfies their intention.
Specifically in the case of the add playlist example given on the article, having it both at the bottom of the playlist list and the header of the playlist section does not detract from either one, but just satisfy different user journeys to get there.
I think you're certainly right when the UI gets crowded -- the MS Office ribbon comes to mind; too many buttons to do formatting including dropdowns that do the same thing as other buttons makes it hard to find related functionality -- "why can I bold and italic here, but not strikethrough here?" because strikethrough was relegated to a submenu somewhere.
I like the discussion of "Poisson" options; thinking about the distribution of inputs is an important factor. For location/country dropdowns, I find it frustrating that often my country, the "United States of America" is put at the top of the dropdown for convenience, but if you miss that and start to scroll down, it does not appear in the "U" section of the dropdown. Just put it in both!