I don't see any reason why this can't be included in 0.18.4 as the original code that is patched has not been changed over the last few releases.
That's not one of the criteria Hadoop uses to determine whether a patch should be backported. That the code is stable is an argument against backporting this for two reasons. First, there are no known issues with it and any changes may destabilize it, requiring more maintenance releases. Second, applying the patch isn't unduly burdensome, neither does it require any special build instruction, so interested parties can apply this themselves. Hadoop's release criteria are conservative, in part because most organizations want to backport some subset of later releases into stable branches.
With few exceptions, the only patches that get pushed into older branches are those that fix behavior that was unintentionally and incompatibly broken in that branch or earlier. If it can be demonstrated that a previous version of Hadoop was compatible with AIX, then the patch can be pushed into previous releases, but lacking that it requires a special dispensation.