The fact that the word "niggardly" is used regularly in novels 80 years old does not make it an ancient lexicon. There are numerous words used in 80 year old novels. The vast majority of those words are used today, in varying degrees of commonality. Niggardly may not be in highly common use today, but it is still used by some, as is made obvious by the fact this thread exists. Again, it is not the duty of the author to aim at a 3rd grade reading level to assure no-one is left gasping at the use of too many big words.
Your argument may have some merit in other situations, such as a professional politician addressing a gathering of working class in an economically depressed area where it can be assumed the average education level is below that of average high school graduate. But the instance being discussed to is NOT another situation. The situation is the word "niggardly" was used by an individual posting on a political BBS. The situation is that one or more people, choosing to be ignorant of the meaning of the word, took the author to task for being offensive.
When engaging in discourse on a BBS, there is no need to "read an audience". It is not unreasonable to assume at least some of the participants are reasonably well educated adults. If there are few exceptions, let those exceptions either learn a few new words, or let them stew in their own gravy of ignorance and emotionalism.
If people are too lazy to look up words they do not understand, too bad for them. It does not take a "professorial type" to take a few seconds out and look up a word. It does take a extraordinary level of intellectual laziness to NOT take a few seconds to look up unknown words. The fact that one is reading a BBS means they are online. How much effort does it take to open a tab and go to dictionary.com?
When writing ideas for others to read, there is some responsibility on the part of the author to write clearly. One does not use language unique to a particular profession unless dealing with others of that profession. You assume the word niggardly is deliberately esoteric. That is not a valid assumption as is evidenced by the author's confusion that it was interpreted as offensive. Just because your only experience with the word comes from old novels does not make it esoteric. Your knowledge, experience and vocabulary are hardly the measure by which the rest of the world be judged.
When reading what others have written, there is also some responsibility for the reader. If a reader runs across a word they do not understand, they have the choice of remaining ignorant, or educating themselves with a quick side jaunt to another online source, taking less than a minute to accomplish. IMO, there is no reasonable defense for those who choose the former reaction to an unknown word.