How did it add to reputation without upvoting in answer?
The answer you linked has one upvote and one downvote, which gives a total score zero, but positive net reputation gain. (Since upvote is +10 and downvote is -2.)
Any user with at least 1k reputation points can see breakdown of the score into upvotes/downvotes simply by clicking on the score.
You can see this also on the timeline of the post https://math.stackexchange.com/posts/1410520/timeline, where you can additionally find out that this answer received an upvote today and a downvote on May 8.
Your total reputation breakdown is available here: https://math.stackexchange.com/reputation