They could have been infected by locals in regions they've been travelling to. Many possibilities. Why do we have to speculate? Let's just wait for more data.
The timeline of HIV pandemic has been established decades after pandemic started. This will take time.
I'd assume that virus originating anywhere in SEA region would be detected in China first. Just like many other viruses originating in America continent would be first detected in USA.
We may have to wait for the fall of chinese communism to know what happened though, like for Katyn. Might take a few decades.
And assuming the chinese haven't destroyed the relevant Wuhan lab samples. That's what I would do in their place. Better never know than take the risk that a technician leak such sensitive information.
The timeline of HIV pandemic has been established decades after pandemic started. This will take time.
I'd assume that virus originating anywhere in SEA region would be detected in China first. Just like many other viruses originating in America continent would be first detected in USA.