Applying strict moral purism to veganism means leaving most of harm reduction out, though, it’s the paradox that happens when moral purism meets lesser-evilism. If the options are getting lots of people to eat less meat by understanding changing cultural norms takes a lot of time and will happen slowly and thus encouraging them to take at least smaller steps towards leaving animal products out (more animals saved = lesser evil), or demanding everyone immediately stops eating meat and becomes vegan, which fails to consider people grown in a meat eating culture will fight aggressively against sudden changes and thus makes people less likely to listen and reduce their meat consumption (more animals eaten = greater evil), a moral purist vegan will choose the latter or do neither (leading to animals still being eaten more than with the first option - so not a harm reduction stance).

As moral purists views it is completely unacceptable to eat animals, encouraging people to eat just a little animals is not an option. If you can accept the lesser evil, you are not a moral purist, and if you don’t accept the lesser evil then you’re choosing against harm reduction