I don't see how Apple could've made it any easier.

1) Select any file.

2) Change the app in the "Open with" dropdown.

3) If you want it to apply to all files click the "Change all" button.

The fact that the upstream HN comment has to explain two identical-sounding options that do different things should help you see what's wrong with the current impl. Your steps don't work with the "Open With -> Options" context menu like they point out, but rather only with the "Get Info -> Open With" options. Not the best UX.

For example, I can imagine if there were instead two radio buttons in "Open With": "Always for this file", "Always for this extension". All in one place.

Doesn't work when Xcode is the default app. I use macdown for mathjax files for math.stackexchange.com, and I have to use "open with" to choose macdown every time.