The test is executed exactly the same way _only_ if each and every one of the 999 person was told the same "he has run this experiment with 999 people before, and has been right every time." before choosing.

Think, if you'd been the first one tested, would the alien have said "999" or "0"? The test is only same if he says "999" to everyone because telling that will affect the outcome.

Now, only way for him to say "999" and not lie is to posses time machine. If he has time machine, he knows what you will choose because he went and watched beforehand your choice.

Thus, choosing the closed box will give you 1 million dollars if telling the number of participants is part of the test.