Earlier quoted context omitted.
It doesn't. Systemd's pid 0 process does not do anything with or about PPPoE. Networkd does. Networkd is a daemon which manages the network configuration and PPPoE makes sense in that context.
But can networkd operate without systemd sitting as pid0? Btw, that very line is why we have these debates. There is no clear way to indicate if you are talking about just the init part or the whole of systemd. This in large part because the name has stayed while the goals have constantly shifted. And sometimes i wonder if the proponents wants it this way, as they can play the "superior" card by constantly "correctin…
Pid1 /must/ be the first process as it's starting all the others; Shipping a network daemon w/ systemd doesn't imply the network daemon is pid1.