Corbato confidently gives the reason but A) doesn't claim to have coined the term, and may not know what the coiner's thinking was; B) at the time may have had a different understanding than some other members of the group - it's not the sort of thing that people have a meeting about; and C) is writing about something that happened decades ago.
Corbato then cites Take Our Word's prior description - people who weren't present when the word was coined, and who openly say they have no idea: "This is so reminiscent of Maxwell's daemon watching his molecules that we can only assume that whoever dubbed these "system processes" had Maxwell's daemon in mind. Unfortunately, we have found no hard evidence to support this."
Then Take Our Word cites Corbato, creating a loop. The only evidence in that loop is Corbato's flawed (being human), prompted (by reading Take Our Word) memory of possibly limited knowledge from decades ago.