I only know of two FOSS federated alternatives to reddit which is lemmy or kbin.
I think lemmy will scalebmore being written in rust were kbin is written in PHP.
And by scaling I am refering to high user count, high traffic, server resoures, server cost.
To my understanding vertical scaling has limits which is the servers limitations as horizontal scaling if infinite since unlimited servers can be added
That’s true but total horizontal scaling is very hard for systems that need some kind of consistency properties