Typically that error means just that - the Prerequisite or Applicability rules for those particular clients are not evaluating as TRUE, and therefore the WU agent evaluates the update as Not Applicable. I'd review those Rules against one of the clients you are attempting to update.
In this instance, some of the prerequisites are expressly described in the package description, and I would imagine are almost certainly the primary contributing factor:
This package requires Apple Application Support 1.5.2 or greater and QuickTime 7.6.8 or greater already be installed.
What this means is that first you will need to publish, approve, and install the Apple Application Support v1.5.2 package (or newer).
Then you will need to publish, approve, and install the QuickTime v7.6.8 package (or newer). (Because QuickTime also requires AAS to be installed.)
And then, the iTunes package will be installable.