It is not necessary to clear dollars through the US, they can be cleared through other financial centers, such as London or Frankfurt. If they had kept all their transactions outside of the US, they would not have been technically subject to US law.
However, they did use US clearing houses, so they're subject.
SWIFT is a messaging system. It is used to send messages and/or instructions between banks, and between client and their banks. The settlement still takes place between the banks.
Let's take a look at the Hong Kong link you provided.
The Hong Kong Monetary Authority ("HKMA") appointed the Hongkong and Shanghai Banking Corporation Limited as the Settlement Institution ("SI") for the USD clearing system in Hong Kong.
"The Hongkong and Shanghai Banking Corporation Limited" is HSBC.
The USD SI is a commercial bank. Each direct participant has to open and maintain a settlement account with the USD SI and USD transactions are settled across the books of the USD SI.
So, to use the USD clearing system in Hong Kong, you need to open an account with HSBC. They then allow you to settle USD transactions with other members of the HK USD clearing system - who also must have an account at HSBC.
All USD on-line transactions are settled real time on a gross basis and across the books of the USD SI.
So, all HSBC is doing is debiting one client's account and crediting the other client's account. Because it's all internal to HSBC, HSBC's USD balance at the Federal Reserve doesn't change. In other words, you can not use the USD clearing system in Hong Kong to settle USD with someone who doesn't have an account with HSBC.
And, if you'd been blacklisted or sanctioned by the US authorities, you wouldn't be allowed to open a USD account with HSBC.
I'm going to take a wild stab in the dark here: You don't have any experience working in financial markets, do you?
I realize that most US dollar transactions take place through the US, I have read that it is something like 95% of such transactions. My point was that it is not necessary for this to be the case, as in there is no law stating that it has to be done so.
Of course, attempting large oil transactions as in the BNP case, while avoiding using the US dollar and clearing houses would be difficult, to say the least.
And no, I am not a financier, but am aware of some basic principles of property ownership.
Since you appear to know about this topic, could you clarify is US dollar transactions settled through HSBC in HK are (in a legal sense) done in HK, or in the US?
However, they did use US clearing houses, so they're subject.