I don't think Sony could afford Activision. Its own market cap is 133b$ USD, which is a lot, but it would still have ended up being a very high-risk buy/merger. I'd be surprised if they ever had any acquisition talks. Activision is so big that only titans like MSFT could afford to buy them outright.
Still, buying a company that's half your size is quite a chunk. And Activision is anything but a save purchase with the recent scandals. I highly doubt Sony would take such a risk.