At this point it's history: because China borders with Afghanistan and has a primary interest in the stability of the region.

Wang Yi (the Chinese Foreign Minister) last July met in Tianjin a delegation of nine members of the Taliban.

He promised support in the reconstruction of Afghanistan and asked them to interrupt links with the East Turkestan Islamic Movement, accused to have perpetrated attacks in the Xinjiang. For Beijing, some Jihadist Uiguri would be affiliated with the organization.

Wang promised to make the country part of the new Silk Road, contributing with infrastructure investments (motorways, hydroelectric energy, ...). This would prevent social, political and terrorist instability, while saving an unpleasant military action.

Saying China is as impartial as Japan, German, and Italy when it comes to foreign affairs involving the US is either incredibly naive or disingenuous. It doesn’t itself prove Chinese involvement, but there’s a clear history of tension between the American and Chinese governments.

Germany and Italy did have troops in Afghanistan, but they worked to keep the peace.

And China has not only a border with Afghanistan, it also has a history of shady dealings with neighbor countries in trouble.