This question has currently only 1 answer that provides sources. Some answers with no sources are highly upvoted. What is going on?
For this specific case: "Hot Network Question" effect.
Based on the question's timeline, it became a hot network question on Jul 4, 22:46Z when there were 2 answers already.
Hot network questions are shown on almost all Stack Exchange sites, so this question got visibility network-wide. Then, visitors who have a rep association bonus (+100 reps) have enough reps to upvote, so they likely upvoted the question and its existing answers (the top 2 upvoted answers).
Meanwhile, the answer with the source was posted on Jul 6, 13:43Z, almost 2 days after. However, the duration for hot network questions is limited to 2 full days only (i.e. until Jul 6, 22:46Z), so that answer only got 9 hours from increased visibility. After that, it returned to votes from the site regulars and natural visitors again.
An answer supported by citations to reliable sources is, of course, better than one that is not so supported. But our policy does not demand such sources, nor does it prohibit upvoting uncited answers. An answer that seems reasonable to people, that confirms with they know (or believe) to be accurate, may well be upvoted. Honestly I suspect most of even our experienced high-quality users rarely check citations to see if they support the answers that they are cited for.
Sometimes people request sources in comments, and downvote based on the absence of sources. Sometimes this keeps an answer from getting lots of up votes. But sometimes it doesn't, and that is a judgement call for each individual user on each individual answer.