Mongo solves it by not being ACID compliant, which is sort of a big deal. This is like comparing apples to watermelon. Essentially, mongo solves this the same way redis does, but you said redis doesn't count as a database.

Also, it is possible to horizontally scale postgres. It's not for the light of heart, but the easiest way is with read only replicas, which is pretty easy (especially with RDS), and read heavy loads are where postgres really shines, and also most apps out there.