You're not wrong, but look at it from a user's perspective. The documentation (https://www.freedesktop.org/software/systemd/man/latest/sd_n...) explicitly says:

    using this library should be preferred in order to avoid code duplication

Then, if you're not intimately familiar with systemd you might wonder which is more standardized and less likely change between the API and the protocol. Maybe you make the reasonable assumption that it's the API.

Then, you look at the reference code and see some reasonably nontrivial stuff that's a bit outside the maintainer's remit to add.

All of that is going to lead people in the direction of linking the library rather than reimplementing from scratch.