[throwaway373438]

That's more or less correct. The relevant Kconfig is:

  if MACINTOSH_DRIVERS

  config ADB
        bool "Apple Desktop Bus (ADB) support"
        depends on MAC || (PPC_PMAC && PPC32)
        help
          Apple Desktop Bus (ADB) support is for support of devices which
          are connected to an ADB port.  ADB devices tend to have 4 pins.
          If you have an Apple Macintosh prior to the iMac, an iBook or
          PowerBook, or a "Blue and White G3", you probably want to say Y
          here.  Otherwise say N.

In other words, the code is only built into the kernel on architectures where it is useful.