“…idealism is mendacity…” Nietzsche proffers us this unique point of view. You ask, how can he equate something so noble as idealism with lying. Well, consider that the idealist is talking not about what is, but what ought to be, what could be, but currently isn’t — in other words, to a certain extent, a fabrication, a will of the wisp…from a certain point of view, a lie, that is, mendacity. His vision may be a noble ohe, but it is not based on current reality. So the issue becomes how far from reality is a particular idealism, which suggests the depth of its mendacity.
With such a Nietzschean point of view toward idealism, the right-minded person needs evaluate each particular idealism to see to what extent it is stretched irrevocably far from reality, for the further it is stretched the greater its mendacity — and perhaps the less its nobility.
A case in point might be the promises politicians make to the public. The self-same politician is hailed as a heroic idealist by certain segments of the public, while other segments rail against him as a scoundrel and liar. Nietzsche would understand completely.