Ok, looks like I found a way to get what I want. Kibana is just showing whatever it gets from ES, and the default for histograms is to sort by key ascending. Luckily the Histogram aggregation supports the order setting with the same functionality as the Terms aggregation, and Kibana lets you add custom JSON to the aggregation.
So, my solution is to add {"order": {"_key": "desc"}} in the "JSON input" field:
Note that it might still not work fully as intended unless "Show empty buckets" is selected and "Extended bounds" are specified, depending on how the data comes in. But that's a separate bug.
(@flash1293 Do you think you could add a link to this "solution" to the original topic? It might help someone)