A Luftnummer may be a flop, but most likely it is something that turns out to be nothing but hot air. Some typical examples:
- A new law that is supposed to prevent something bad, but entirely fails to do so, because it can easily be circumvented.
- A promise that has never been intedended to be kept in the first place.
- A venture that does nothing but draining money from its investors.
- Something that is claimed to be a success but actually was a total failure.
Note that in the example you quoted, the quotation marks are actually wrong, since they imply that the opposition already used the term Luftnummer with quotes (vorwerfen already establishes that Luftnummer does not necessarily reflect the opinion of the author, so there are no quotes needed for this purpose).
Also note that Leo does not give to turn out to be a flop as a translation for Luftnummer but for sich als Luftnummer erweisen, which is a verb.