I believe Karma solely comes from upvotes for comments minus downvotes. Submissions don't count.

That might be in real life ("afk"), but on HN even submissions give you karma.

Have a look at your submissions, they brought you karma. <https://news.ycombinator.com/submitted?id=exagolo>

Although nothing is crystal clear, the karma system is not 1:1 for submissions.

<https://news.ycombinator.com/item?id=29024032>

    Comment upvote +1
    Comment downvote -1
    Submission upvote >0 && <1 (not documented to prevent abuses)
    Submission downvote not possible (only flagging is allowed)