Funnily enough, IBM had already published this. CUA explicitly says tab and backtab move between fields.

so they spent seven layers of management escalating against their own standard: https://archive.org/details/ibmsj2703E/page/n13/mode/2up

Not being familiar with the term CUA, I looked it up and TIL something (https://en.wikipedia.org/wiki/IBM_Common_User_Access). Since the doc you linked is dated 1988 and CUA was a brand new thing circa OS/2 (at least in traditional IBM timescales), the apparent inconsistency might be down to the massive scale of IBM in those days and organizational propagation delay.

Another factor could be still-reverberating echoes of the likely political battles around something as broad and far-reaching as CUA. I can only imagine the quiet boardroom battles won and lost fighting over CUA between different factions across all of IBM's kingdoms, divisions and principalities.