That is a wrong view. Iran's position is that it has full sovereignty and does not allow any negotiations. When China supports the two sides' negotiation — which is the UAE's position — it has already chosen the UAE's side. There is nothing more to discuss about this.
Keep in mind that neither the UAE nor Iran is an ally of China. The only country with which China has a mutual defense treaty is North Korea. The reality is that, compared to Iran, China places greater importance on its relationship and interests with the UAE.
Just look at the reactions of both sides and you will know which side China is on. The UAE readily agreed to issue a joint statement with China, while Iran reacted with anger and rejection. Just look at how these two countries responded — you will know exactly which side China is on.