Being a citizen and being a tax resident are still two different things, at least when it comes to countries. Voting is based on citizenship while things like a driver's license isn't tied to citizenship in most countries, you can actually get a driver's license in any country you reside in for that time.
The US has many double tax treaties with other countries for people that are in your situation. Check this for the country you are living in. I am in a situation like you and don't pay in my native country. I pay taxes in the country were I generate the profits. But I am not american...
In his situation of living abroad in Hong Kong...there's no taxes there. The real issue is would he have to pay taxes on his profits to the U.S. government because the broker he's using is a U.S. broker. Simply, the U.S. may not see his income (profits) as income abroad considering its a U.S. broker and he still maintains a home address (he called it a billing address) in the U.S. while he may not have legal residency in Hong Kong. That legal residency of Hong Kong thing could be the sticky point considering nothing is in his name in Hong Kong...the U.S. may not consider him living abroad especially with him maintaining a U.S. home address. Regardless, seems like an easy answer via him calling the IRS in the U.S. about this issue. I know I wish I had done so to prevent the audits many years ago. Luckily the country I moved too has a law that if you're paying taxes in the country...you've establish "legal residency" for taxation purposes....qualifying to be exempt from taxation in the U.S...later I establish "legal residency" via immigration. No more confusing taxation issues after that.
I think you guys are conflating the OP's question about state taxes into answers about federal taxes. There's no question that U.S. citizens are subject to U.S. tax regardless of where they physically live or how they earn their money. The U.S. has tax treaties with many countries that allow you to deduct any taxes you paid to that country from your U.S. tax burden. However that in no way relieves you from filing out a full U.S. tax return accounting for every dollar you earned and the tax you paid overseas. This is straightforward and clear. The state tax question is much less clear. When you move between states in the U.S. you generally no longer have to pay taxes to the state you left on any money earned after you moved. However the mechanism to "prove" this to the losing state is proof that you filed returns in the "gaining" state or that you are a resident of that state by some other means. Residency isn't usually clear cut, it's more of a preponderance of the evidence kind of thing which I know from my time in the military where this was kind of a big deal (the law allows you to keep your residency if you're active duty even if you're in another state). Everyone agrees that just having a mailing address doesn't make you a resident. Everyone agrees that having an address, a driver's license, registering to vote, and actually spending a significant portion of your life in a new state makes you a resident. Doing some but not all these things leaves you in a gray area. The consequences of being in this gray area are entirely dependent on the aggressiveness of the "losing" state. If you're in a high tax NE state, I know from experience that they'll force you to prove you've changed residence or they'll claim you owe them taxes and the onus is on you to prove otherwise. Remember, they know you paid last year so you're in their records, and they get a copy of your federal tax return so they know how much you earned. If you're in TX, FL, WA, or AK where there is no income tax, they probably don't care if you're in Hong Kong, Hawaii, or Timbuktu.
IRS has 2555 foreign income exclusion for people living abroad 330 days a year, I would think there is something similar in the state tax code.