There are many reasons, explained repeatedly, to suspect the outcome of this poll. All Musk has to do the show it was legitimate is conduct the investigation you outlined, and maybe explain why most of the world didn't know a poll was being conducted until it was over.
What are the chances of this happening?
BTW: You also suggested that mail-in voting could not be trusted. Explain to me how you would commit voter fraud using absentee ballots. You can't, for the simple reasons that it is difficult, time consuming, risky, and with a very low probability of affecting an election.