Why would it not be? What data is it illegal to view? Other than perhaps CSAM, which I would strongly hope Disney don't host on their Slack.
I think at least some legal systems agree with my interpretation, but the U.S. is insane.
I did mention that I considered the U.S. interpretation insane, didn't I?
IANAL, but I think this description is overbroad. There is a "work for hire" doctrine in copyright law that assigns copyright to the employer, but I believe by default that only applies to works of authorship within the scope of an employee/contractor's assigned duties, with any broader scope needing to be explicitly assigned by contract. I would expect internal communications in general to be covered by an NDA or some concept of privacy rights, depending on the context.
Have you ever worked in entertainment?