Quote - Yes, completely 64-bit Cocoa applications have been possible since 10.3.9.

No. 10.3 did not allow any 64 bit applications whatsoever.
10.4 allowed command-line only applications on a very limited set of system calls (system.framework and accelerate.framework).
10.5 finally allows to write 64bit GUI applications, and Apple didn't give a definitive answer to what APIs are available in 64bit until the day 10.5 shipped to end users. And that's why you don't see any 64bit applications for OS X - the only two that I'm aware of are Xcode and Chess (and none of the two benefits from 64bit).