I'm a white, U.S. born, citizen and I read it the first way not the second. The context of "Hello Londoners" makes it pretty clear.
It will be clear to anyone who’s heard of London’s black cabs (which I’m sure is a lot of people) but it’s probably damn confusing for those who haven’t.
I'm not so sure. I didn't know what London black cabs were and I still read it as (black cab) driver. I just assumed black cabs meant something like black car cabs, e.g., Uber SUV.