Postgres has a query planner, I mean this entire post is about improvements to it. So I think there either was some miscommunication or your friend doesn't know

Postgres has a query planner, I mean this entire post is about improvements to it. So I think there either was some miscommunication or your friend doesn't know anything about Postgres. There are very large Postges databases that seem to work fine, so Postgres can certainly scale. But SQL Server also has some features that Postgres doesn't, and if those are important for you it might work better for your use case. They are in the end different databases with different strengths and weaknesses.