Apple never promised Java 6 for Leopard.
Apple didn't even "as much as" promise. Reading the blurb for the WWDC session you linked to, they promised "resolution independence, a crisper Aqua look and feel, a 64-bit virtual machine, and more", all of which are delivered as part of the Leopard's bundled Java 1.5.
What happened was that a bunch of people who didn't know any better assumed that Java 1.6 would be in Leopard, despite the fact that Apple never said it would be. And after talking up that release date for months, they forgot that they'd made it up themselves in the first place!