The quoted text is in the context of what one can get with existing UNIX tools. The paper goes on to claim that short of severe operating-system bugs, the result is that the process has no communication channels other than its initial file descriptors.
Does anyone dispute this claim, or have a better suggestion of how to implement that result with standard UNIX interfaces?
(I haven't looked into the bug mentioned, though I'd guess that isolate(1) is a world-executable setuid utility that has a bug allowing arbitrary code execution before doing the setuid step, rather than being a bug in the approach quoted from the DJB paper. Anyone who's looked into it, please either confirm or refute this guess.)