It doesn’t make sense from a Ukrainian POV, but it does make sense for the Americans.
Let’s just assume their wet dreams come true regardless of how utterly insane the odds are against it, Ukraine attacks Moscow during the parade and injures or kills Xi. China declares war on Ukraine and PLA troops start to pour into Ukraine en mass and starts to absolutely crush the already battered Ukrainians. America can also use the attack as a pretext to end all support for Ukraine as Trump wants.
Chinese rapprochement with the EU is dead, and NATO becomes more important than ever to the Europeans with the prospect of a massive Sino-Russian army rapidly approaching their boarders in a vengeful mood to boot.
NATO rush troops to the boarders as a safeguard and one false flag attack against those NATO forces later, NATO is at war with China and Russia in Europe. America can now kick off their long dreamed of re-enactment of the 8 Nation alliance to go to war against China by dragging all of NATO into the fight. Most of whom would join in gladly under the circumstances.
The point has never been about whether America might try such an attack using Ukraine as proxy, but rather it doesn’t have the raw hard power ability to pull off such an attack against the kind of security that will be in place. And both Russia and China will be on maximum alert for such an attack precisely because of the massive geopolitical implications a successful attack could potentially immediately bring forth.
Rather than waste time and bandwidth talking about an event as likely as aliens landing and wiping America off the face of the earth to curry favour with China, it would be far more interesting and relevant to explore the possible fallout from a failed Ukrainian attempt to attack the parade.
China would be well within its rights to consider that as an attempted assassination on its sitting head of state, and thus and act of war by Ukraine. Indeed, if the intention is to try to bait China to directly participate in the war in Ukraine, irrefutable proof could easily be planted that Xi is the intended target.
Best case for the Americans, China declares war on Ukraine and it’s the same game plan as before. Worst case they can use that as a pretext to cut Ukraine off and basically extricate themselves from that money and weapons black hole without looking like the bad guys, and also pull the EU closer to it due to the increased security threat to them, while at the same time it can package its ending of support for Ukraine as an olive branch to China to end the trade war.