When has Trump ever "showed sincerity" to China? Trade War 1.0, HK colour revolution project, African Swine Fever, and Covid-19. Each of those events coincidentally happened during or after negotiations between Trump and China.
The days when China thinks that a deal could be reached with Trump is over. Today, China is in a "bring it on" mode with Trump and America. China is going to punish Trump this time and make him crawl back begging, or let him rage impotently in the Whitehouse. Those 6th gen aircraft test flights are an appetizer for what is to come.
Indeed. Trump has always just given meaningless empty words of flattery with his usual limited vocabulary and meaningless trivial gestures like chocolate cake, then gone on later to make racist derogatory remarks or take actions that are meant to hurt Chinese interests. It's as if he truly thinks other people will fall for such shallow short term flattery. A similar pattern is seen with his relationship with Kim Jong Un, where empty blather and shallow gestures didn't get Trump or the US the unilateral North Korean nuclear disarmament they were after and their talks in Vietnam collapsed.
The fact also his first trade demands in order to avoid tariffs amounted to demanding China give up its economic sovereignty, amounting to a total surrender of its economy to American approval, shows how unrealistic he is about actually dealing in good faith. If the demands were genuinely made then they were fools to think China or any country would accept them as it would be political suicide for any politician. If the demands were made with the intent that they would be rejected and the trade war start, then it shows Trump and his team were never honest about dealing with China in the first place and were always looking for a fight.
With Trump it's the Art of No Deal, because his deals are always demanding basically you surrender to him in an abject humiliating way.