Assuming that this was not just a simple mistake (it was not), I could assume:
(1) he has a US passport which states this is his birthdate and the employee simply copied the birthdate from the passport. Why would he do that? Could it be that the different birthdate would cause a mismatch in the IRS system and prevent me from locating his assets as a resident alien?
(2) he used that birthdate intentionally, even though it was not on the passport.
We are making two assumptions: (a) he wants to hide assets, and (b) he has the ability to be a resident alien, should he so desire.
A: In my opinion, the only reason to make a misrepresentation on an I-9 is because the applicant is not really eligible for employment in the USA (i.e., an illegal alien). So, the subterfuge is not intended to defeat your search, but rather the search of Department of Homeland Security -- Immigrations and Customs Enforcement (DHS-ICE).
An alien does not have to seek resident alien status. An alien does have to be eligible for employment under either a nonimmigrant or immigrant visa, if the alien is physically present within the territorial boundaries of the USA. An alien can be the CEO of a corporation without a visa, as long as the alien is not physically present in the USA. And, an alien CEO can receive compensation/wages as a nonresident, once again, as long as he/she is not physically present in the USA without a visa.
Regardless of all of the above, if the CEO is a party to the case, you can notice the CEO of the corporation to a deposition to appear and testify and to produce all of the corporation's financial records. That's how I would handle the issue.
I think you may be trying to uncover a fraud by an unnecessary methodology.
Hope this helps.