Is the lesson for native-chinese-speakers about speaking English? If so, interesting, I haven't actually seen this (presumably common?) oopsie before. It also hadn't occurred to me how crushing of an insult it is, but damn yeah it sure is.
A "thing" or "stuff" in Chinese is 东西 (dōngxi). That's literally "east-west" if you pick the individual characters apart. That's what the footnote refers to.
Calling somebody 不是个东西 (bùshì ge dōngxī) means something along the lines of being good for nothing, i.e. an insult. Translating it literally, it would be calling somebody "not a thing".
Saying someone is irrelevant is likely an insult, also in English.