That's because it is a broad spectrum source; it includes a lot of red, but it looks yellow/white because of all the other frequencies that are also present.

So if you need to be exposed to just red light it won't work well, but if the red light works regardless of the presence of other colors then the sun seems like it would be a fine source.

It still contains that wavelength.

> Most of the solar radiation that reaches Earth is made up of visible and infrared light.

https://www.fondriest.com/environmental-measurements/paramet...