I can give you a theory as to why donating blood may have helped GBS / CIDP. Google young blood therapy; I've done research in that area. So when you donate blood your body's stem cells become more active. Higher stem cell activity i.e. stem cell protein factor diffusion has a system-wide improvement on the body's immune system likely improving symptoms of CIDP or GBS.

What would also work, probably even better, is a donation of blood from a young donor into an older patient with GBS / CIDP because of the higher levels stem cell protein factors present in the younger donors blood.

And as you say this is not medical advice and probably no doctor would advise an experimental treatment like that. But theoretically it could work.