UV is the one to worry about, fortunately it's easy to filter, for example most clear safety plastic glasses don't let it through.

Could that be harmful to the eye?

Yes, just like looking at an eclipse without the proper gear. Your eyes don't register the threat and stay wide open while getting blasted by harmful wavelengths

Would that somehow be different, for 3 minutes, than being outside in sunlight for an hour? Are you maybe thinking of ultraviolet light?

It can be different yes. As far as I know, wavelengths that you don't perceive will not cause your pupils to contract in response to harmful intensity of light.

While it can be very bright in the open sun, your eyes will compensate for this. A light source which emits harmful amount of IR and little to no visible light will probably not elicit this reaction and damage your eyes without you realizing until it's too late.

Infrared lasers are particularly dangerous for this aspect.

This is also how you get snowblindness on cloudy days. The light intensity isn't high enough to cause your pupils to dilate the same way as on a sunny day, and the snow reflects a bunch of UV into your eyes. So you get a sunburn on your retina.

Yes but the reason I pose my question is that the topic is red LEDs, not IR LEDs nor laser diodes, and there's nothing in this article suggesting that for some odd reason you must stare directly into the light source as opposed to looking at a diffusor.

I insist that the "OP" is mostly being alarmist without thinking about all the 620-700 nm (and deeper infrared) exposure we perceive throughout every day.