Yes, Aqua uses the so-called INTERNALDATE from the IMAP server.
This is intentional -- using the Date header from actual messages can produce wrong results when the sending device has misconfigured date/time (happens sometimes). I actually used to get a lot of reports / complains on this.
Some mail services will set the INTERNALDATE to the Date header when you upload old messages -- apparently, your mail service is not one of those.