They're definitely not comparable, but having been involved in implementing dark mode for an iOS app (the quote you're reffing is talking about the mobile app, not website): Yes, implementing dark mode for at least an iOS app can be surprisingly challenging (or at least, a lot of work). It's probably much easier to write a brand new dark-mode supporting app, but retro-fitting it onto an existing app is a whole other thing.
Your designers need to re-colour the entire UI (and this turns out not to mean just "Map colour X to colour Y" btw), you need to cope with on-the-fly switching (iOS devices can be configured to switch dark mode setting automatically at sunrise/sunset), anywhere you're doing custom drawing needs to have dark mode retrofitted into it, and if you're getting imagery from a backend anywhere you'll probably need to add support for getting both light/dark mode images at once (which of course also means you need to re-draw all that imagery for dark mode as well). There's bound to be more stuff I'm forgetting.
So yeah, dark mode for an app can be a bit of work, it's not just swapping colours.