Because he works there. He has the most information possible. He is not guessing. The SEC is quite clear about how this has to be honest disclosure. He has been

Because he works there. He has the most information possible. He is not guessing. The SEC is quite clear about how this has to be honest disclosure. He has been rebuked several times for this. It is not unreasonable for companies to miss targets and when the forward looking information they provide at investor briefings may be less trusted and this would normally be reflected in the stock price change. Experienced investors make long term investment decisions with this sort of thing in mind. Not everything goes to plan but if this was fabricated it would be illegal and is very commonly pursued by the authorities.