This is not the first time he has said this. When tesla stock was hovering around 300$ for the first time back in many years ago, he repeatedly said the same thing. The stock corrected some 9 months or more later.

If he believes the stock is high, he is trying to calm the hype behind it. And not look bad when the stock actually self corrects.

isn't the entire market down today though? how can it be proven that his tweet impacted the price versus a broad market loss?

you can do this with a synthetic control. i did this at https://www.alexpghayes.com/blog/elon-musk-send-tweet/

Very nice little article. While my kind might not be the target audience would you mind adding a short ELI'mNotAnEconomist explanation?