But there doesn't seem to be a single Han marker. There is a mix of markers associated with the Han, but none are unique. They're each shared with different other populations is if they've been in intermittent trade with the Near East since the second millenium BC or something.
If you want to bioweapon China and just China none of the markers are specific enough. You'll get better than 10% of most of southeast Asia. The US can't do that, the whole reason we might want to wipe out China is to protect our allies in the region who would also be susceptible to the same plague, especially the Republic of China. Russia can't very well do it either. It would hit the Manchu, Kazakhs, Mongols, and Vietnamese. The Kazakhs are Russian. The Mongols and Vietnamese are Russian allies. The Manchu I believe straddle the Sino-Russian border. Also, it's going to upset the US due to their large Chinese population and many allies in the region and Russia without nukes can not afford to anger America while in a war with China even if better than half of the Chinese are going to die. The ones that don't have that marker are still very numerous and would be completely immune.
This is representative of issues with targeted human diseases generally. Humans travel for trade and warfare and either stay and settle or leave bastards and that's not counting full on migrations.
The solution then may be to pick multiple markers that intersect with enough of the theoretical target population to ensure over a certain percentage target kill/offtarget kill. You'd need pretty extensive intel on your target populations genomics as well, which I imagine isn't freely available. If you wanted an effective 'ethnic cleansing' bioweapon, you also would't go with just one. You'd probably go with a dozen. Ones that target various intersecting genome markers that will hit large amounts of the target population; phages that disrupt the functioning of nominally safe local bacteria - most phages can act as reservoirs for HGT, so you could induce toxicity in gut bacteria, or non harmful foodborne bacteria - bacteria that destroy crops, viruses that kill animals common to the region. Which would require yet more intel, more money, more time, and more people in China to deploy them; and then yet more people to monitor the effects of said bioweapons.
Even the most 'lethal' diseases can be contained, and firebreak plans would definitely be used to halt any spread of one disease; vaccines can be produced, the kill rate can't be too high or too fast since it'll burn out before being effective. You'd need to breed in anti-viral and anti-biotic resistance. Plus every one you develop has the chance to turn around and infect you as well. Then you're got to think of the point of it all, if you just want to kill your enemy; then conventional ballistic missiles can still be very large. You could load payloads of things like nerve gasses, or just normal HE; and keep hitting enemy cities/military bases/ports/etc with them until their capacity for war is destroyed. None of those have astronomically high R&D costs that developing a dozen death plagues would have either. Even putting aside the strategic concerns, or moral concerns; I doubt that you'd be able to get enough bang for your buck when compared to just old fashioned chlorine/sarin/VX/TNT bombs dropped onto the enemy. Hell, nukes may not work, but radioisotopes are still lethal. Airbursting dirty bombs would probably work better than a theoretical bioweapon.
Any theoretical America that did try and develop a tailor made plethora of diseases to deal with 'The Han Question!' would be so far down a moral rabbit hole that they'd be rimming satan. So I imagine that there'd be plenty of issues regarding that on the home front as well. Because the USA that does shit like that, definitely has internment camps for Chinese Americans.
I domt think we have that kinda technology
We don't. We have the bare infancy of maybe that technology. You'd need to engage in some pretty dammed unethical experiments before we'd have that technology honestly. It would take a lot of bodies; money, and researchers that could be doing literally anything else besides LARP'ing as Pre-War Fallout scientists. People always talk about CRISPR-Cas9, I've worked with CRISPR-Cas9, it's not particularly accurate. It's probably one of the
most accurate and powerful gene editing tools we have today, but it still has lots of errors, offsite mutations and such.