Not a definite answer, but some observations and interpretation:
The train is planned to end in Hamburg-Harburg and to not go to Hamburg Hbf. It does that until August 12th already where you're automatically offered a change to S-Bahn. On your date, the cancellation has not been properly added to the schedule yet, and only a short (manual?) information has been added. That might be added properly or clarified at a later time.
Another possibility is that the message has been added by mistake and the train will go all the way to the main train station. In any case it won't be a huge problem. There are several other options to get from Harburg to the main train station, e.g. the local train S-Bahn which goes fairly regularly and doesn't take much longer.
There are some hints for the reason you see a strange message:
- There is no hint for a cancellation in the machine-readable data, i.e. the database that is used to create itineraries and schedules.
If it was added there, the last stop would be marked as cancelled properly and you should see an itinerary to use the S-Bahn between Harburg and Hamburg
That seems to imply that the message has been added manually.
- The train one hour earlier has the same information - albeit having the more sensible "between Hamburg Hbf and Hamburg Altona".
So I guess this is just a minor editing issue due to someone using the same message template for both trains without noticing this one ends in Hamburg Hbf.
My first interpretation which seems to be wrong on further review:
The problem might also be a hiccup of the software due to the fact that you can't enter the train in Hamburg-Harburg (note the "Exit only").
The template of the warning message likely has two blanks: One for the first station you can't enter the train for an onward journey and one for the last stop that isn't served. And, in this particular case it turns out that both blanks are filled with "Hamburg Hbf".
This logic will work out in most cases, but just not in the one with one stop missing after the "exit only" stop.