I am fairly sure what you wrote is not true. Under federal law for a federal work center (does not cover state employees), if a union covers a federal work center, all employees of that work center are forced to join the union.
https://nlrb.gov/rights-we-protect/rights/employer-union-rights-and-obligations
“The NLRA allows employers and unions to enter into union-security agreements, which require all employees in a bargaining unit to become union members and begin paying union dues and fees within 30 days of being hired.”
JoMa
I am personally, 100% sure that what I wrote is most definitely true.
:)