It is certainly not wholly in good faith, but it is not in totally bad faith either because china really is the only power big enough to take on this role, that russia could possibly accept. So when EU said only china can do it they are right. Of course they hope china won’t gain from it,I agree that China can potentially gain from doing so. However, this call by the EU and US is most likely not done so in good faith. The US would try to sabotage any Chinese attempts at brokering a deal and then pin the blame on China. At the very least, the US would love to see the Chinese brokering a deal which involves states being allowed to secede from a sovereign nation by popular referendum. Before you know it, the US will be calling for popular referendums in Tibet, Xinjiang, and Taiwan.
but mark of effective diplomacy is to get a lot more out of an opportunity offered in bad faith than the offerer had anticipated.