Not surprised. What could a job at either Valve or Bungie give him that he doesn't get now?
To be more precise: Working at a larger studio like Valve means you trade some creative control and (in notch's case) some short-term income for reliable long-term income and access to things you could never get in the short term as an individual, like access to devkits for consoles, access to well-trained, highly skilled QA/art/programming departments, decades of design and programming experience from leads and senior developers, etc.
Hey I f'ing love Valve, my all time favorite game company. No one is trying to suggest working for Valve is a bad thing. But a little perspective is needed here.