Catalysis of the reaction: 5-amino-6-(D-ribitylamino)uracil + 4-hydroxyphenylpyruvate + 2 S-adenosyl-L-methionine + H2O = 7,8-didemethyl-8-hydroxy-5-deazariboflavin + 2 5'-deoxyadenosine + 2 L-methionine + oxalate + ammonia + 4 H+.