It's a hard-coded (as you found, compile time macro) value that resolvd uses when it can't find any configuration. So, one doesn't need to patch the source or recompile resolvd, one just needs to add a configuration file (what most distros do).

That said, it's mind-blowing that systemd needs a DNS resolver to start its boot sequence. It should really be able to run without one, and have this as the default. It should also not try to set the network up before it runs the network setup code, and respect all of the user's network configurations. The entire thing is completely messed up, but this one complaint isn't all that it looks like.