Correlation and causation - look up the difference (not just you, but all the dipshit reporters should too). China's economic growth was always going to decline. There's little doubt that tariffs are having some impact, but mainly slowing growth is based on a structural trend as China transitions to a middle-income service economy. You can't keep up the same level of GDP growth the more advanced your economy becomes.
Also, how the fuck does slowing Chinese growth help you? America's growth is slowing too - and that 'slow 'Chinese growth is 3 times America's growth. Trump could just nuke the entire world and kill everyone. That would slow China down even more.