There are actually situations where U.S. citizenship can be a requirement, e.g., cleared jobs but there needs to be a really good reason like not being able to obtain clearances.
To conform to U.S. Government export regulations, applicant must be a (i) U.S. citizen or national, (ii) U.S. lawful, permanent resident (aka green card holder), (iii) Refugee under 8 U.S.C. § 1157, or (iv) Asylee under 8 U.S.C. § 1158, or be eligible to obtain the required authorizations from the U.S. Department of State. Learn more about the ITAR here.
ITAR, defense, and clearances are completely unique worlds compared to the rest of the job market.
That said, if Elon used Tesla employees for Twitter business, what to say that he doesn't do the same for SpaceX business?