There are legitimate reasons for moving the functionality of an external program into a library and using the library directly. Unfortunately, the reason for a PPoE library is not given. If I had to guess, it probably has something to do with running on an embedded platform. Or, possibly, the normal pppd binary requires too much of an operating system than systemd can provide at the point where networking needs to be set up.

These are my guesses. Your guess seem to be that Lennart is unaware or unwilling to consider orthogonal design?