Activision game is closed source and I never expect the source code to be released for it and over time hackers found vulnerabilities. I also do not expect Activision to make patches for older games. Plutonium may patch these COD vulnerabilities today but I am certain there are more vulnerabilities in Plutonium which will be found someday weather it is open source or closed source.
If Plutonium is open source, hackers as annoying as they are can work for you as they become your free cyber security researchers by trying to hack Plutonium and finding vulnerabilities for you.
Look at many FOSS software, heck look at Linux itself. Is Linux a security nightmare because it is open source were Linux users fear being hacked on a daily basis? Nope! Linux is the most secure OS out there because it is open source and vulnerabilities are found and patched.
By open sourcing the project, it will become much more secure in the long run as long vulnerabilities that are found get patched.