Maybe Jaeger was keying on the castle, the torches, the fireplace, the armor, and the swords and buckler for his initial assessment. Missed or did not recognize the bust of Napoleon.
After all, the dialog references to the 18th century come after the 900 year exchange.
Yeah. I think the whole thing can be rationalized (however unnecessarily) by saying:
- Trelane's planet is 900 light years from Earth.
- Trelane has seen human culture from as recently as his most recent artifacts and music indicate.
- Our heroes made a simplistic assumption about a "viewing scope" based on their complete failure to accurately estimate the age of Trelane's many cultural artifacts.