Of course you can not improve the system by adding a uncorrelated losing strategy, but you can improve the profit by adding a losing strategy with negative correlation. Look at the following equity curves:
A: ___/'''''''''''''\____/'''''''''''
B: /''''''\_____/''''''''''\____
A is winning, B is slightly losing. Add both curves and I hope you'll see then what I mean.
A: ___/'''''''''''''\____/'''''''''''
B: /''''''\_____/''''''''''\____
A is winning, B is slightly losing. Add both curves and I hope you'll see then what I mean.

But for the sake of theory, just assume that the strategies are not overfit and the equity curves are from real trading.