Why does a higher stroke volume reduce angina?

Stroke volume. Stroke volume is the amount of blood ejected from the heart with each beat. A higher stroke volume implies increased cardiac contractility which means more oxygen demands by the heart muscle. In the presence of significant coronary artery disease, this increased contractility will increase, not reduce, angina.
Angina. Higher stroke volume helps decrease angina by decreasing the oxygen demand on the heart and improving the oxygen supply and demand ratio.
CO is increased. Cardiac output is increased and hence the "supply" side of the supply-demand equation is increased.
Stroke volume. It doesn't higher stroke volume means the heart is working harder likely to provoke angina.