That's not exactly true--any junior programmer could single handedly write a Twitter clone with a bad backend that doesn't scale.
Conceptually the structure of Twitter is easy, but like anything of the sort there's a lot going on that isn't obvious or even visible to the user. And more to the point, even if it really was that simple, building a platform that can support hundreds of millions of user interactions per day requires a lot of people with an uncommon skillset and a huge amount of experience using it.
You're right that those things aren't trade secrets, though--they're what you get from years of working at a company doing those tasks and honing that skillset.
It's like building a modern car--it's conceptually simple, but there's a lot of moving parts there and a lot of backend that make it happen efficiently, so actually executing the process isn't anywhere near as easy as it seems. Also like designing a car though, anybody can take the thing apart down to the last bolt, so there's no magical trade secrets involved in the process, it's just that actually executing it well is something that requires a lot of talent and experience that you mostly gain by doing.