The field of epigenetics looks at types of modifications that a cell makes to its DNA, without changing the coding for genes. Such modifications may increase the translation of certain genes or may inhibit the translations of certain genes, changing the programming of a cell. One of these types of modifications is called DNA methylation.
The chemical process of methylation simply refers to the addition of a "methyl group" to a given molecule, such as DNA. A "methyl group" contains one carbon atom and three hydrogen atoms. In the methylation of DNA, a methyl group is added to a specific location on the DNA, which generally inhibits the translation of that section of DNA into RNA. When a gene's translation is inhibited, there will be fewer copies of that gene's RNA found in the cell.
While it is theoretically possible that a low blood concentration of the main methyl donor S-Adenosyl-Methionine (AdoMet or SAMe) will lead to reduced DNA methylation, widespread reduced DNA methylation was not noted in the paper you mention.