Well, of course it’s voting. So was student council elections in grade 11, and so was student union elections in my second year of university.
But CCC 2240 talks about “Submission to authority and co-responsibility for the common good”. You need to vote because these people will be using your tax money and voting on laws, but primaries are not about who gets into power. It’s about who gets to be the nominees for the particular parties, and parties are private groups. You definitely have to vote in November in the general election (and in any other special elections or whatever), but a primary doesn’t really count because it doesn’t change anything in the government (directly). So-and-so gets nominated to run for the Democrats, but they still have to win the actual election to take the seat. Like I said before, it’s more of a vote within a private organization.
Like if someone were a shareholder in a company, I would say it’s not morally obligatory to vote in the board of directors election (it’s probably a good idea, but not obligatory). It’s a private organization, not an authority.