Value =/= cash reserves. Valve has ran VERY lean for quite some time.
If Valve's secrecy is so good that they have (substantially more than) 30-500x cash stashed away in excess of their public valuation estimates, then perhaps I underestimate Valve's secrecy!
More likely, it was an obviously-humorous exaggeration, but I wasn't sure -- I am quite ignorant of the games industry. :)