Actually, looking over more carefully, tjefferson looks more right than I realised. 1900 pitchers are averaging .536, and 1910 pitchers are averaging .513. That's not bad.
19th century hitters are averaging an incredible .548, but 1900s and 1910s hitters are all under .500, so it isn't really true that deadball hitters as a group are that impressive, just 19th century hitters.