Catalyzes the oxidation of methyl-H(4)MPT to methylene-H(4)MPT. 5-methyl-5,6,7,8-tetrahydromethanopterin + H(+) + oxidized coenzyme F420-(gamma-L-Glu)(n) = 5,10-methylenetetrahydromethanopterin + reduced coenzyme F420-(gamma-L-Glu)(n) Metabolic intermediate metabolism; lactate oxidation. Belongs to the mer family.