Had the same question when we played it some time back. From my personal AAR it seems as we had received an answer that ground conditions were also mud and thus played it that way but I can't seem to find a notation to the official response among my notes. (One of my FAV scenarios BTW)
Mark's Desperation Morale site has a nice "Slopes" play aid for download. Once you've played a turn or two with slopes it becomes second nature and easy to handle (the first turn or two you may be baby-stepping a bit). Barbed-wire fences are just too easy (though I believe a bit overdone).
I don't believe the rule was ever changed, it was simply clarified though there was some errata that issued that expounded on the principal that Intensive Fire wasn't allowed if marked with a Final Fire counter..
It's kind of difficult to remain HIP (and most cases concealed) when you have to fire at a unit entering your Location; A8.312 TPBF: An armed, unbroken Infantry DEFENDER not in Melee must after all Residual-FP/minefield/OBA attacks then immediately attack any Infantry/Cavalry MMC unit that...