To the extent which people with ME/CFS are undermethylated, I think it would result from insufficient methylfolate and/or B12, both of which are susceptible to oxidative stress--which, to my knowledge, is the best explanation for our collective problems with them.
There is some debate about just how undermethylated we are, or even how to tell. Some people, like Freddd, encourage people to take high amounts of methylfolate and methylcobalamin (and some other stuff) and to push through "overmethylation" symptoms, which in at least some cases eventually go away. Other people suggest that you can know how "methylated" you are based on your reaction to methylfolate and methylcobalamin. For example if it gives you insomnia, you probably are overmethylated. Still other people, myself included, think that because of the brain's ability to adjust to increased methyl groups, symptoms may not be the best yardstick eg. if a tiny amount of methylfolate and methylcobalamin gives you insomnia, maybe it is because you have been starved for methyl groups but maybe it is because you already have enough.
I am less familiar with histadelia. I see that high copper is blamed for histapenia (low histamine in the brain), so I wonder if low copper might sometimes be a part of histadelia? It also makes sense to me that they would suggest one stay away from folates, insofar as folic acid will compete with methylfolate for entry to cells and once inside cells will gum up the works and slow down formation of methylfolate. In any case, it seems like undermethylation is only one possible cause of histadelia.