Understanding why my shower hob is so high
The hob, or curb, lined with that rubber membrane you see there, exists to contain water. Imagine your drain gets a little clogged with hair like shower drains will do, and it starts draining slowly and water stands in your shower. It’s just there to provide a buffer of safety. 

The membrane is in place to catch water that gets under the tiles and bring it to the drain, which it will, because grout and mortar are porous materials. Your plumber was probably referring to this. 

The second photo, what you were hoping for - that bathroom likely has a rubber membrane under the entire bathroom floor, and the entire bathroom floor is likely pitched towards the drain.