passwd is accessed by multiple programs and fast lookups are a reason to implement binary formats. Same for termcap, timezones, etc (though I wouldn't call those configuration files...). Postfix might also need fast lookups and doesn't keep in-memory state.
For a service manager however these reasons do not apply. Having it use a binary configuration database makes no sense as there is only a single program accessing the configuration that doesn't need fast access to these. And no, I don't think claiming that it's just "compiled for runtime use" makes this much better[1]: it doesn't help with having the proprietary binary format and its source go out of sync, requires special tools to debug (rescue systems might not have s6-rc's compiler installed[2]) and so on.
[1] That's like saying the Windows registry is just "compiled for runtime use" from *.reg files ;)
[2] Yes, one might be able to use the compiler via a chroot, but why should I have to spend more effort?
For a service manager however these reasons do not apply. Having it use a binary configuration database makes no sense as there is only a single program accessing the configuration that doesn't need fast access to these. And no, I don't think claiming that it's just "compiled for runtime use" makes this much better[1]: it doesn't help with having the proprietary binary format and its source go out of sync, requires special tools to debug (rescue systems might not have s6-rc's compiler installed[2]) and so on.