I didn't say he regretted anything, just that unscrupulous people seized on the perceived theme of apathy and have tried to weaponize it ever since.

Edit: from this op-ed

W.E.B Du Bois (1868-1963) was an African American civil rights activist, sociologist and philosopher. He developed a theory of how one should vote tactically. Crucially, Du Bois did not equate voting tactically with voting for the lesser of two evils. In his essay I won't vote (1956), Du Bois outlines his general strategy for how to cast your vote. You should:

• Research who best represents your interests. Go with the candidate, not necessarily with the party (in Du Bois' case, he looked at the extent to which a candidate was willing to help the cause of African Americans)
• If none of the main candidates represents your interests, you should vote "for a third party even when its chances [are] hopeless."
• "If the main parties were unsatisfactory; or, in absence of a third choice, [you should be] voting for the lesser of two evils."
• If there is no third choice, and you are deeply dissatisfied with the candidates on offer, it is acceptable not to vote. This was controversial, especially given Du Bois' earlier insistence on tactical voting. Yet, Du Bois believed this could send a strong signal "It is hope that if twenty-five million voters refrain from voting in 1956 because of their own accord…this might make the American people ask how much longer this dumb farce can proceed without even a whimper of protest."

So Du Bois' understanding of tactical voting is much richer than merely voting for the lesser of two evils (although he did think it was sometimes necessary, see (3)). You don't always vote to change the outcome. You may also wish to vote — especially in a safe seat — to give a signal. Refraining from voting also sends a signal, but needs to be done only in extreme cases where you have not a single acceptable candidate and all candidates are equally bad.