The only definition I've ever known for the term "sweetening" is one of purification and sterilization. In other words, bringing the pipe back to a neutral state in which the full, natural flavor of the tobacco may be enjoyed. Some pipe restoration specialists use ozone chambers to "sweeten" a pipe; certainly an ozone chamber doesn't make the pipe smoke like a stick of liquorice.
It's more of colloquialism in the tobacciana vernacular by this point, but I see no point in reinventing the wheel.
I would never literally "sweeten" a pipe. I never use rum, whiskey, or any other flavored spirit during the cleaning process. When I clean pipes, the purpose is to sterilize the chamber, destroy bacteria, and rid it of lingering tastes -- not introduce new ones. I (and the people I've restored for) want to taste the tobacco, nothing more. That's why I use full blown Everclear (the 190 proof variety). Is it overkill? Possibly. But it may very well be the strongest product on the market, and I'd rather tend towards excess than risk a mediocre restoration.
In addition, briar does not like excess moisture, especially for prolonged periods of time. The higher the alcohol content, the faster the chemical reaction, the faster the evaporation rate... meaning less risk to the pipe.
In over one hundred restorations, I've never once had a person complain of a ghost flavor or otherwise nasty taste, regardless the age of their pipe or estate purchase.
Anyway, that's the reasoning behind my approach.
Edit: I just realized that I left out "sweetens" under the 91% Rubbing alcohol method. As per the aforementioned definition, it does just that. That was an error of silly omission on my part and may have led to some confusion as to what I considered "sweetening."