That's clearly just a rumour, 16 billion is probably about the revenue valve makes in a year, there's no way it can be bought with this value to begin with, Valve's easily worth hundreds of billions of dollars. Honestly, there isn't a lot of companies in the world who could even afford to try and buy them.
Plus it's certain gabe doesn't want Valve in the stock market, so as long as he is alive and well even a 600 billion offer will probably just go by. companies need to wait for him to die and then try to buy Valve from his successor and we can only hope that he has the same core beliefs as Gaben.