I would certainly believe that donating blood could cause a short-term increase or decrease in HPPD symptoms, just by virtue of changing the amount of blood in circulation - it changes things a little. I'd also believe that a short-term decrease in HPPD symptoms could help take the persons mind off it, and lead to some lasting improvement.
By "I'd believe" i mean, the theory isn't ridiculous, that it's plausible - not that I believe it just based on that.
HPPD symptoms can be made worse by all sorts of things, so why could something like this not reduce the intensity of symptoms? The idea is plausible, but the poster's claims are most likely exaggerated if based on reality.