The leaked call reveals little novel information. Nuland was already having public meetings with Yatsenyuk, Klitschko, and others on the eve of the revolution. It's a foregone conclusion that such meetings were to exercise US influence to promote the US foreign policy position on what ought to happen if the government was to get toppled in a revolution.[1] This isn't evidence that the US instigated the revolution, or that the US was a big part of the factors behind the revolution. It's "US-backed" in the weakest sense of the US having communicated their desired outcome if the revolution came to pass after the ball was already set in motion by Ukrainians.

That's why we shouldn't label it a US-backed coup. That label connotes a large degree of involvement and influence over the causal factors underlying the revolution which can't be supported.

If the US followed Russia's tactics and poisoned pro-Russian Ukrainian leaders, or sowed disinformation in their media ecosystem, then sure, the label of a US-backed coup would be more than appropriate.

[1] Actually, this is me being overly generous. While that is one possible interpretation, another is that they simply recognized that Yatsenyuk will take power due to his overwhelming popularity among Ukrainian people, and are simply trying to preempt and manage the inevitable factional politics that could arise post-revolution.