Language is a tool, you have to do what's best for your own goal.

If you read Orwell, his message is not necessarily that complex language is worse at transmitting ideas, as he's actually arguing that complex language can hide the speaker's real motivation and deceive more easily.

For Paul Graham, I'd say for him the 'write like you talk' is very good advice since he's interacting with founders whose first language is not English, people with different backgrounds from his, young folks that maybe didn't take an academic route, so for him it checks out to recommend it.

Leslie Laport always talks about how you should always write down what you think. Until you write something down, you only think you're thinking. Also, he's all about writing most things in math over English, since math is less ambiguous (and less complex). And I'd say math is quite different from how you talk.

Now, you can notice how you can have different motivation for the same behaviour (Orwell and Graham), or different behaviour for a similar motivation (Orwell and Lamport). Maybe more interestingly, think about people with the opposite intentions from the ones above: a contractor that wants to mimic sophistication to get a contract with a bank (with representatives also mimicking sophistication); guilds trying to preserve a high barrier of entry. The advice they'd appreciate would be the opposite since their goals differ.