The law is called the "Railway Labor Act" which was amended specifically for airline mergers in 2008 under a law called McCaskill-Bond.

Here is direct language from the Law:

(4) the term “covered transaction” means—
(A) a transaction for the combination of multiple air carriers into a single air carrier; and which
(B) involves the transfer of ownership or control of—
(i) 50 percent or more of the equity securities (as defined in section 101 of title 11, United States Code) of an air carrier; or
(ii) 50 percent or more (by value) of the assets of the air carrier.

Here is the specific section on the Congress website.

https://uscode.house.gov/view.xhtml?...edition=prelim

Its clear there is no requirement for United to bring over pilots on asset purchases if they don't break the 50% threshold. Its not "covered" under this law, which is the only law that mentions requiring integration of other airline employee groups.