These are general guidelines. Arrangements like these are arranged country to country bilaterally. And some are enforced and some are not.
And if you have a diplomatic passport for Taiwan for example doesn’t do you any good when you go to Japan except that you get to go through the diplomatic immigration line. Other diplomatic considerations they give you are mostly as a courtesy for a diplomat but not necessarily guaranteed.
And diplomatic bags and pouches when transiting third countries could or could not be treated as a diplomatic pouch. Most countries will follow the General guidelines but they could open it and do whatever the hell they want and suffer the consequences, if any, later.
Then depends how the bag is carried. Is it just a guy carrying a bag or does the guy have an official documentation saying he is transporting official diplomatic material to a specific location.
The whole diplomatic passport and transport of diplomatic bags can get quite nuanced.