I just found that on Mac, Obsidian has smart punctuation toggles in the menu bar, in the standard Edit > Substitutions menu. I thought the app just didn’t support the system smart punctuation setting, but it turns out the smart quotes toggle in that menu was turned off. The setting doesn’t exist in Obsidian’s Settings area; I’m guessing maybe it comes with Electron (the framework Obsidian is built in).
I’ll also note a thing I already knew, which is that Obsidian on IOS follows the system setting for smart punctuation.
For now I’m still using the Smart Punctuation plugin on Mac because when you wrap existing text in quotes (by selecting it and typing a quote), the app only makes one of them curly. And I’m using it on iOS so I can have smart quotes without smart dashes (which interfere with HTML comments).