Could be, but it's not always so easy as that. We don't know how they coded the diplomacy functions. It could end up being very simple, or a somewhat major overhaul. Even the simplest additions to code can introduce bugs of their own as well.
Perhaps they have a function which determines the overall value of the trade, along with a gpt (any per turn item, luxuries and resources along with gold) flag, which gets checked against the players rep with the AI to allow for, or disallow, the trade in whole. Now they'd have to write code to individually check each item for if it's gpt, because before they were only flagging a trade with gpt, not an item. They'd need to flag each of those items, and then subtract the value of those items from the overall value, or just add up the value of non-flagged items. This might require a change to diplomacy item objects to allow for these flags, while the flag for a trade object would be removed. There could be multiple areas in code which need to be altered or to be adjusted.
There are numerous other ways that diplomacy could have been implemented, and some of them definitely could be easy to change. When they first implemented this check, I'm sure they did it in the simplest way they could think of. To me that seems to suggest that it might not be as easy to change as you think.
That aside, it's a petty bug which has no real affect on gameplay. I actually like it, because it gives a very quick barometer of where your trading reputation stands. Even if it only took 30 seconds to fix, then that's 30 wasted seconds.