Not sure it's sanctions or tariffs, just there're $307 billion worth of machine equipment per year they cannot buy directly.

Did you account for value addition? It may be HK is in the middle of electronics value chains that start and end outside of its borders - I imagine Singapore has a similar import/export numbers it makes a lot of hard disks (export) but doesn't make a lot of chips (import). This doesn't imply Singapore is a front for another country

There isn't much of electronic industry in Hong Kong since the 80's. https://en.wikipedia.org/wiki/Manufacturing_in_Hong_Kong