I always understood it as statically linked binaries that could be used regardless of the state of the rest of the system.

Both /bin and /sbin are generally statically linked. So you're not wrong; but primarily sbin is intended to mean "executed by root" above all else.

Looking through my /bin and /sbin, none of the files are statically linked.

Hrmm what distro? I haven't looked in sometime, maybe something changed (Slackware here). My understanding was static linkage in boot related folders was to minimize chance of accidentally losing a library/dependency and breaking booting.

Looked at both Debian and Redhat.

How can one discern whether a file is statically linked?

file(1) or ldd(1)

I understood it to be 'secure binaries', locked away from regular users through security policies.