In general, that term is mostly used outside of the borders of a country looking in. After all, "illegitimate leaders" tend to be authoritarians who take power and quell dissent within the borders.

Not at all arguing that it somehow leads to justification for an illegal invasion.

In this specific case the claim comes down to assertions of a sham election. If this was indeed the case (with the lens of an international survey obviously the US view is suspect considering the attack), then the Venezuelan people themselves do not view him as a legitimate leader, which simplifies the situation.

Ah, but then who can?

I think my assumption that the legitimacy of a government rests in the eye of the beholder is pretty reasonable.