SCL-IPC is a domestic Chile service. The rule I quoted appears to apply to all domestic Chilean services, without making any exceptions. If a OW ticketing agent quoted that rule to me I can't see how I could make any case for travelling in anything other than Economy, regardless of my class of ticket.

But again if you have actually travelled on that route using a DONEx (or AONEx) and not been in Y then I accept that the rule is open to different interpretations.