Your name here seems quite appropriate, as that was a weasel tactic, rephrasing the opponent's words to modify their intent. Not to mention you also used a quite inflammatory tone towards someone who's strongest words were "I strongly disagree".
I never saw any assertion that the user must manage apps individually, only that they should be able to. Easy way to do that - by default, the app echoes the system setting. If and only if the user manually changes the setting, does it diverge from that. Somewhat simple way around having a persistent setting get stuck - if the user switches back from the opposite mode, then you go back into "system" mode. So really, instead of needing two toggles, you would have one that has two states - "Respect system setting" and "Use the opposite mode from the system" (which probably need to be worded way better). The only snafu there that I can think of, off the top of my head, would be that if the user changes the system setting, the app state would likewise change.
It's also completely valid if someone doesn't wish to use system-wide dark mode (say, due to specific applications they use that may not function or render correctly in dark mode) but yet wishes to have specific apps in dark mode. I run Windows, which does have a dark mode nowadays, but that only works for certain apps that specifically use it (Windows Explorer does, not that I use that, along with many of the "modern" style apps that ship with Windows), and yet in apps like Discord I still prefer to use the dark mode, and I also have a dark mode extension for my browser.
For what it's worth, the site did correctly detect my dark mode setting in Windows, and display the dark style. As well, I don't necessarily disagree with any developer that does wish to exclusively use the system setting. However, I do see room for both approaches.
