I mean can't one buy those IR emitting LEDs for security cameras or TV remotes for cheap? Seems like they'd get the job done but it's hard to say what intensity is required.

The exact wavelength on the red/IR spectrum matters for how deeply the photons penetrates into cells. IIRC, 600-1100 nm is a sweet spot, so from deep red to IR. But the same wavelengths don't seem to help all cells equally. I think eyesight is improved from 600nm-800nm, but skin and muscle healing with deeper IR wavelengths. Interesting stuff.

Note though that near infrared also carries a risk of cataracts. I think they used to call it ‘glass blower’s eye’ because looking at the furnace was a high dose of near IR. Modern IR heaters use much longer wavengths.

Good point. I'm sure it has a lot to do with dosage and intensity. If near IR stimulates more activity in mitochondria (the purported mechanism here), that indicates there are some proteins or other compounds sensitive to that wavelength. It then stands to reason that large dosages at high intensity over prolonged periods can stimulate so much as to cause dysfunction, aka glass glower's cataracts.

I play around pottery kiln, you have to be careful looking at red hot stuff.

You usually don’t need to look at the light source as it penetrate the skin, clothes, skull, etc.