Define “usual people who compile this package”. In my books those are distro maintainers who indeed should be familiar with options provided by the init system they choose. If you decide to include oddballs who compile everything themselves (Linux from scratch?) then again, yes, they are expected to know a thing or two or dig into options or even code from time to time.
In this case it should be assumed we’re talking about someone who’s compiling systemd and familiar with the source. And yes, these are flags that anyone who builds systemd from source would be familiar with and know how to set.
It’s the same as when you run a configure script and pass —-disable-feature.
Furthermore, even if you compile it, you don't have to use it. Even if you install it, you don't have to use it. (After all it has 2-3 APIs, one of them is the /etc/resolve.conf symlink, if you instead just put 1.1.1.1 in that file, libc will not go through resolved. And similarly for the other entry points.)
It's possible to accidentally compile, install and have systemd-resolved set up on your computer, but it still seems rare that someone can compile and install systemd, yet not know what they're doing.
I ask not rhetorically, just out of curiosity. It's not a typical type of suggestion for other packages I've encountered with open source, and it feels far more difficult to discover than typical decisions at compile time.