That was actually my first thought when I saw the screenshot with the numbers near the top of the screen. Slice that part off, stream the rest.
A similar thing shows up on YouTube a lot, actually. People heavily crop uploads of copyrighted material to get around their content identification stuff.
I wonder if they also use something like Digimarc for a non-visible watermark? They could publicly credit the visible watermark as a distraction from the real technique.
That's exactly how it works. The numbers are hashed by the receiver and uniquely identify that particular viewing card, then displayed in a position selected by the channel operator, when triggered at a time of their choosing.
If one were so inclined, I imagine it could be done with dedicated hardware specifically made for generating the overlay. May not be worth the cost though.
The smart card has nothing like the power required to process video - but it doesn't need to. The Sky receivers are fortified sufficiently to make changing the firmware a distinctly non-trivial operation.
Even with only two channels, I expect you could binary search it down to a single receiver over the course of a single season of matches.