At the very least GP included a some sort of source (a link to an interesting wikipedia article). You dismissed the source with an anecdote. Not sure you're the one who should be making claims about pseudoscience.
GP's source doesn't actually motivate why an after-image is tiring for the eyes.
I'd associate eyestrain with the muscles in the eye. Like you can give yourself eye-strain by quickly moving the focus from close to far-away a couple of times.
Meanwhile, after-images are a neurological artifact. They do not cause your eyes to change focus, and are not caused by the muscles in the eye.
Although extremely bright light may strain the eyes in the same way your eyes may be sore from contracting if you go outside on a bright summer day.
Regarding the pseudoscience argument, maybe this helps a bit: as far as I understood it, there are two types of after image: One is a neurological artefact, the other one is a biochemical one.
What I described earlier as exhaustion is buried in a process called visual cycle, which is a biochemical process. If enough photons of a given wavelength hitting special molecules of a part of your retina, these molecules run through a replenishing cycle after photon absorption to run through the same loop again and again. If light source is strong enough, the cycle is not adequate to account for the incoming mass of photons, essentially triggering temporary deactivation. The molecules responsible for colour vision register the inverse of the incoming wavelength, as in: the absence of all other wavelengths registered gets interpreted as the colour one sees. This essentially creates the yellow after image if one looks at an object with strong red and blue content.
This is a can of worms and there is more involved, even on the side of the emitter which is why there is measurement in watts per steradian going on but my knowledge ends here. Maybe a Biologist can chime in.
