That's true, but most sites don't support dark mode anyways, and it's still not clear that dark mode will be widely adopted web designers. In any case, you're right there would just be a second font file for dark mode, so at least a user wouldn't have to download both.

In any case, as of two months ago Chrome still explicitly has no plans to implement SVG in OpenType (despite support in every other major browser, and the request being 3 years old), so color icon fonts don't appear to be coming to the web as a whole anytime soon. :( [1]

https://bugs.chromium.org/p/chromium/issues/detail?id=306078