It’s only silly now in retrospect when the PT tells us Obi-wan is such a dummy that he’s basically saying “…only a master of evil, Lord”
The word “lord” is a weird case where you wouldn’t really use it in that way (more likely “my lord”). I always saw Darth as a title used more like “general” or “master” or “count,” where people refer to them by just their title all the time. I’m not a linguist so I apologize if I’m not explaining this sufficiently, though it should be easy to understand.
That makes sense. If Darth is nothing more than a designation (like Moff or General), Obi-wan is saying that he’s no more than this designation, a mere cog in the Imperial machine.
This still doesn’t explain why he says ‘A young Jedi named Darth Vader’. I don’t think there’s a way of interpreting this as anything other than his actual name.