You appear to have a rather rigid definition for a fat tree. Generally however, a fat tree is any tree where the links get fatter the more you approach the core.

A skinny tree is a tree where all links have the same bandwidth/speed. Any tree that has some speed increase towards the root can be called fat. Fat trees with no oversubscribed uplinks, impossible to be congested are rather rare.

You appear to have a rather rigid definition for a fat tree. Generally however, a fat tree is any tree where the links get fatter the more you approach the core (or the root).

A skinny tree is a tree where all links have the same bandwidth/speed. Any tree that has some speed increase towards the root can be called fat. 

Fat trees that have no oversubscribed uplinks and are therefore impossible to be congested are rather rare - you'd need a 50 Gbit/s uplink for a branch of 50 ports with 1 Gbit/s each.