I read the article as saying that there's no official C library but unofficial ones do exist. Quote below, emphasis mine.

> A official c library doesn’t exist yet unfortunately, but there’s several out there you can try.

Also, it looks like there is more than zero support for C programs calling Landlock APIs. Even without a 3rd-party library you're not just calling syscall() with a magic number:

https://docs.kernel.org/userspace-api/landlock.html

https://github.com/torvalds/linux/blob/6bda50f4/include/uapi...

https://github.com/torvalds/linux/blob/6bda50f4/include/linu...