Yes, that's the difference with rp-pppoe.so (using kernel module to handle encapsulation, userspace code only sets up the sockets) and pppd's pty option (got it a bit wrong in my other comment in this thread, believed it's a pipe, thanks for correcting) with separate process to do so. It doesn't really matter.

More than this, Linux kernel has kernel-mode PPP. But still a few parts of PPP, and all accompanying control protocols (especially authentication-related ones like EAP - you don't want that beast in the kernel!) are done in userspace.

Would not be the first time systemd has overloaded something kernel related...