Yeah, this is really concerning. The handwaving around "keeping the ui up to date" by hosting it on ui.duckdb.org instead of embedding it doesn't taste great to me.
At least it's hosted on duckdb.org and not mother duck, but I really would expect to see that source somewhere. Disappointing unless I've missed it.
Breadcrumbs in the extension src: https://github.com/duckdb/duckdb-ui/blob/963e0e4d4c6f84b2536...