On Line 308 of Nyx.cpp, the value of do_hydro is used in the following if statement :
if (DefaultGeometry().isAnyPeriodic() || (!do_dm_particles && !do_hydro))
However, this if statement comes before the actual call to read_hydro_params(), which sets the value of do_hydro given the input file. I suspect that !do_hydro above always evaluates to true because its default value is zero.
If I am correct, then && !do_hydro should be removed from the if statement.