Yes it really jams. I know the difference between an actual jam and a reporting error. Once the jam is cleared from the duplexer, the next page goes through with no issues (after, of course, the C7280 goes through its routine of preparing itself again). And everything recommended at the link you provided has been done multiple times without any change in the frequency of jamming.
Refer the section in the document to 'reload paper' to make sure that the paper is being loaded correctly.
Finally, the last section in the document has suggestions to prevent paper jams. Ensure that you are following these suggestions.