That argument assumes that the delegation of a capability to another process must happen through a path of interprocess communication that can be established only by the operating system, if the processes that want to communicate have the capabilites for this.
I have not studied to see how the existing capability-based operating systems solve this problem, because it seems that this is not a simple solution. If the capabilities are very fine-grained, to make certain that IPC really cannot happen, that might be cumbersome to use, while coarse-grained capabilities could be circumvented. To really prevent IPC without appropriate capabilities, a lot of the convenient features of a UNIX-like system must be forbidden, like the existence of files that can be read by any user, or directories like /tmp , where anyone can write a file.
I have not studied to see how the existing capability-based operating systems solve this problem, because it seems that this is not a simple solution. If the capabilities are very fine-grained, to make certain that IPC really cannot happen, that might be cumbersome to use, while coarse-grained capabilities could be circumvented. To really prevent IPC without appropriate capabilities, a lot of the convenient features of a UNIX-like system must be forbidden, like the existence of files that can be read by any user, or directories like /tmp , where anyone can write a file.