Somewhere in the middle. Some other KDFs folow a similar concept (or even construction); but there certainly are many KDFs that do not, at all. So, basically, we can't expose it anyway at a high API level, but we can at the concrete level here :)

(Most KDFs either cope with IKM being terrible by being hard to compute, like scrypt, or by defining an input step, like HKDF, or by handwaving and saying that your input key should be good, like a bunch of ad-hoc KDFs in a bunch of RFCs.)