I see reading comprehension isn't your strong point. The flight was not oversold because the 4 employees did not hold a confirmed reserved space, so BY LAW, United could not deny boarding to any passengers holding a confirmed reserved space to accommodate their own employees. It's really that simple.
Go read 14 CRF 250. That's the only rules that apply in this situation. Any policy by United contradicting this is in violations of the law and therefore not valid.
link:
https://www.law.cornell.edu/cfr/text/14/part-250