You are totally correct.  Continuing to report the current status as 30-late informs others reviewing your CR that the account is currently delinquent.  Thus, it is significant.

It is not necessarily conclusive that reporting a $0 balance signifies satisfaction of the debt.  If a creditor sells a delinquent account, they would also report a current balance of $0/

They apparently have not read the CRA credit reporting manual, titled the "Credit Reporting Resource Guide," in which current status is clearly defined as a snapshot of the account status as of their last reporting, and does not reflect any prior, historical status. 

A satisfied account is no longer delinquent.  Its current status is Paid.

The 30-late would be reported, as your stated, separately as its Payment Rating code, which retains record of the level of delinquency prior to its current status.

I would advise them that they have a statutory requirement (FCRA 623(a)(2)) to update any prior reporting so as to maintain its accuracy.

Their prior current status of delinquent is no longer accurate.