That's not an actual exception but a note clarifying what the GPLv2.0 covers.
Linux’s exception is because you need its header files to compile c code that uses the kernel. If someone created an extension that provided a json api to blendr, it seems that would need to be open source but callers would not since nothing would need to be compiled against gpl code.
> NOTE! This copyright does not cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does not fall under the heading of "derived work".