Catalysis of the reaction: S-adenosyl-L-methionine(1+) + acetyl-CoA + 18 H+ + 8 malonyl-CoA + 11 NADPH = S-adenosyl-L-homocysteine + 8 CO2 + 9 CoA + dihydromonacolin L + 6 H2O + 11 NADP+.
acyl-CoA:malonyl-CoA C-acyltransferase (decarboxylating, oxoacyl- and enoyl-reducing, thioester-hydrolysing)