First, thanks for sharing this link, it was an interesting read! A few remarks below.

I had a hard time reading the wc code in the article. First I had to go to the GitHub to understand that "da" stands for dynamic array, and then understand that what the author calls wc is not at all the wc linux commands, which by default gives you the number of lines, words, and characters in a file, not the count of occurrences of each word in the file, which is what the proposed code does.

Also, since I had to read the GitHub README, another remark: it says that sp_io uses pthreads rather than fork and exec. Both of those approach (but especially pthreads) are contradictory to the explicit goals of programming against lowest level interfaces. I believe the lowest level syscall is clone3 [1], which gives you more fine grained control on what is shared between the parent and child processes, allowing to implement fork or threads.

[1] https://manpages.debian.org/trixie/manpages-dev/clone3.2.en....