Catalyzes the demethylation of diphthine methyl ester to form diphthine, an intermediate diphthamide biosynthesis, a post-translational modification of histidine which occurs in translation elongation factor 2 (EEF2). diphthine methyl ester-[translation elongation factor 2] + H2O = diphthine-[translation elongation factor 2] + H(+) + methanol Protein modification; peptidyl-diphthamide biosynthesis. Interacts with INCA1. Belongs to the DPH7 family. Truncated C-terminus.