You could use a similar approach to what the WP Discourse Shortcodes plugin does. It makes API requests to Discourse to get data in a JSON format, then uses the data to generate HTML.
It’s been a while since I looked at that plugin, but possibly it could be extended to add a leaderboard shortcode.
I’m going to make some Discourse blocks for the WordPress Block Editor soon. I’ll think about creating a leaderboard block. Leaderboards are a bit of an anti-feature for me, but I don’t think that’s the case for a lot of people.
Discourse could also look into creating more embeddable widgets - similar to Embed a List of Discourse Topics Into an External Site. A leaderboard widget would be a good candidate for that.