I don't know how they do it, although the reddit commenters do mention IR emitters/reflectors in the ad panels being used to create a mask.

What I did notice is that the system commonly has very slight glitches around player bodies if you pay very close attention. This is most visible in close-up shots from mid-height cameras. The effect is similar to how video call blur filters imperfectly detect hair boundaries. The ad boards sometimes cover small parts of players' faces.