This is wrong, on Windows there are system calls to access memory of other process and on Linux you can do it using debugging. Also on Windows there is a tradition to inject libraries into other processes, create threads in processes etc.
On Linux, ptrace permissions can be restricted [0] and some distributions do this by default.
Whether this provides any meaningful security is questionable unless you pair it with filesystem isolation to prevent malicious programs from modifying config files / bashrc / etc. Meanwhile it does make legit uses of ptrace more annoying.
