Red light therapy is one of those things that sounds like total woo hokum when you first hear about it, but that studies keep showing benefits to.
Key thing to remember is that these studies are using very high doses of red light - often limited by the amount of heating you can safely tolerate in the subject (not heat from the emitter, but from the photons themselves).
Honestly, it doesn't sound crazy at all. Different tissues have different light susceptibilities (that's how, say, laser hair removal works), due to the susceptibility of specific compounds. Red penetrates very deeply (remember the classic trick of shining a flashlight through your hand; only the red makes it through). Red light has a number of very specific interactions it causes. The main one is that it photolyzes the bond between NO and cytochrome c oxidase (triggering NO signaling, causing vasodilation). A secondary possible affect is that it affects light-gated ion channels (esp. in the TRP family).
And the amount of red light we're talking about is very high. Instantaneous fluxes are like 1000-5000 times higher than ambient indoor light in the red spectrum, and even averaged across a day it's still like an order of magnitude higher than a person who spends their day indoors. For NIR therapy, the difference is even more extreme (commercial LEDs emit essentially zero NIR). While outdoor light exposure is much higher - 15 mW/cm2 red + 20 mW/cm2 NIR, comparable to the 10-100 mW/cm2 of red light therapy - it's broad spectrum, while red light therapy is concentrated around cytochrome c oxidase peaks, so the flux around these specific peaks is ~10-50x higher with red light therapy.
Flux matters a lot because to meaningfully dissociate NO from CCO, you have to do so at faster than the rate that NO and CCO recombine. Sunlight on its own (let alone indoor ambient light) don't do that. It doesn't matter if you're spending all day in the sun vs. 15 minutes in treatment if the sun isn't hitting the threshold effect level.
Another difference, BTW, is not just "shooting it up your nostrils", it's scattering. Sun just hits your skin and much of it is lost through scattering and absorption in blood, esp. hemaglobin. Red light therapy targeting deep structures is delivered by a contact probe that compresses the skin, displacing blood. This increases penetration into deep tissues 2-5x.