The kernel supplies /dev/futex functionality.

glibc supplies the pthread API that uses nptl on top
of /dev/futex functionality.

db-4.1.24 uses the the pthread API, has run-time check
that API is functional. The runtime check requires
    a) the running kernel supplies functional /dev/futex
    b) glibc with nptl.

So, rpm carries
    Requires: kernel >= 2.4.20-0.pp.9
    Requires: glibc >= 2.3.1-15
attempting to insure that the platform on which
rpm runs has (at least) the necessary capabilities available
to use posix mutexes through nptl.

This, of course, is totally bogus if, say, a different
kernel w/o /dev/futex functionality is booted.