Atomics International was a division of North American Aviation (NAA) when the reactor incident occurred in 1959. The reactor was located in NAA’s Santa Susana Field Laboratory (SSFL) located in the hills between San Fernando Valley and Simi Valley.
Rocketdyne (also a division of NAA) also tested rocket engines in SSFL in a different location at the time. Atomics International was merged into Rocketdyne much later.
Got it: It was not Rocketdyne that ran the reactor.