I agree with everything but I think this is a weak line to use:

a single vote is not going to make a difference

it's true that leftists are too small in number to sway an election, but with that line you're just gonna get "what if everyone thought that way." You can see how the lib you were talking to latched onto that one line and ignored everything else.

more importantly, the whole purpose of denigrating voting is to get people to organize. As long as people organize, whether or not they also vote in national elections is of little consequence imo, as long as they have realistic expectations. If they think there's some marginal harm reduction, that's fine, as long as they don't pin all their hopes on some crisp, bloodless Democrat who'll let Citibank pick their cabinet like Obama did in 2008.

People need to understand that, even when the majority votes blue, their votes do not actually result in policy. We have to break the false sense of political agency that voting gives people. But the purpose is ultimately not to stop people from voting, but to make them start organizing.

Testing Theories of American Politics: Elites, Interest Groups, and Average Citizens (2014)

Multivariate analysis indicates that economic elites and organized groups representing business interests have substantial independent impacts on U.S. government policy, while average citizens and mass-based interest groups have little or no independent influence.

[...]

In the United States, our findings indicate, the majority does not rule — at least not in the causal sense of actually determining policy outcomes. When a majority of citizens disagrees with economic elites and/or with organized interests, they generally lose. Moreover, because of the strong status quo bias built into the U.S. political system, even when fairly large majorities of Americans favor policy change, they generally do not get it.