I think it is because now he can threaten to quit at any time if he doesn't get what he wants (payraise, bigger cubicle, bigger coffee mug, private bathroom, etc), and the management will scramble to make sure that he stays in the company. As long as he keeps his mouth shut about the critical knowledge he can ride that gravy train forever.
Then again, knowing Wally like we do, there is the good chance that Johannsen didn't really tell him anything, or told him that there is no critical knowledge (what I like to call a "Kung-Fu Panda situation"). In either case he can now do whatever he wants whenever he wants.
When I first saw this strip, I didn't see Wally as angry. What I see is Wally with a maniacal grin on his face because now as the sole holder of the "critical knowledge," he must be lavished upon, and can never be fired.