I love that they're discussing this, and kudos to them, but unless the specific details are described (e.g. "and there are lots more log lines than dashboards and users" - what does that mean? That dashboard_id is nullable and many rows log entirely different things? That would be a significant problem unto itself. What is the cardinality of users, dashboards, and sets thereof), it's hard to really take much from it.

As a general recommendation I advise database developers to avoid exactly this sort of "outsmart the planner" behavior because it paints you into a corner -- data cardinality changes, schemas change, and you have large, clever queries that turn into a significant net detriment. They can absolutely be necessary in some cases, but this does not look like one of them.