Generally speaking, there is no virtual "init ABI" facility for other packages to depend on (that the alternative implementations would be declared to provide exclusively). Therefore any package must declare its applicable alternatives for the ABI it requires, and in this case the libguestfs0 packager knows only systemd-sysv | sysvinit-core as alternatives. Perhaps the required dependency is only satisfied by those, or perhaps the maintainer has not investigated runit-init as a third alternative.
To pursue that further, you should lodge a bug to Debian regarding libguestfs0 about it.
For yourself, you could create and install an equivs package for, say, sysvinit-core and hope for the best.
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