No. I don't always assume that just because a CEO makes a public statement they have internal data backing up the claim. Sometimes they do! Other times, they have data but are misinterpreting it or missing something, but it's impossible to tell if the data is just internal. Other times they're making a statement without data based on their personal beliefs. Other times, they don't even think the statement is true but are saying it for messaging, marketing, or communication reasons!

Like the previous commenter, I'd be much more confident an asymptote was reached if it was being demonstrated publicly.

Only OpenAI and its CEO know the full details on GPT-4's sizes so that's entirely possible. But since it's an internal secret, there's nothing compelling him to tell the truth. For all we know, he has internal data backing up the opposite of the claim but is making this claim so as to discourage potential competitors from spending the money training an even bigger and competitive ML model. Sending potential competitors off on a wide goose chase that, when pushed, he can just say "oh our internal data (that no one outside of a trusted few have seen) said otherwise". I have no idea if sama is such a person, but you must admit that the possibility exists.

You'd hope so, but unless people put their evidence in public, it could simply be a tool to manipulate the public's expectations or competitors behavior.

I'll get downvoted for this, apples previous CEO was consistently inaccurate about company innovation and performance numbers.

He might or might not, this is a point of the claim.