Unfortunately, the universe is not so kind. Antonin may or may be systematically trying out every bar in the city, but if not, he is sampling a wide swath. He's starting relatively near Severus' place and working outward; this is a repeat visit to this particular venue.

Severus had told Antonin that Rodolphus is in the city, and Antonin has seen that Bellatrix is. It isn't difficult to add two and two. He hadn't sought Rodolphus out before, as Azkaban has left neither of them the better, but not seeking him out and ignoring him are two different things.

He slides into the empty seat and says, "You look like a man who could use a refill."