Stock price should include risk. Just like a lottery ticket might be worth $0.5 and costs $1 to buy that doesn't mean that after the draw it's not worth buying it off someone for $10 dollars.
In this case I'd expect that he is also suggesting that he believes the stock to be overvalued on typical ways to evaluate stock but from what he believes the company will actually accomplish he thinks the value is higher.
Note that I'm criticizing this beliefs, and not the distribution of future probabilities. Of course company might be overvalued now at $250, and sure, it might hit $500 later.
But if you're certain it will be $500 in three years, you need some serious risk aversion (~2500bps) to think it's not worth buying it now at $250.
In other words, by saying it will grow to $500 you're expressing your view on value; this value is higher than price, so stock is undervalued.
Of course it can -- the company's real value could easily close that gap in three years. Also, equity pricing is as much about psychology as it is about real value. A stock's value is equal to what people are willing to pay for it.
In this case I'd expect that he is also suggesting that he believes the stock to be overvalued on typical ways to evaluate stock but from what he believes the company will actually accomplish he thinks the value is higher.