It would not surprise me if they knew about it before that, though I'd need to see whether what they knew included what Alsop revealed in his article, which referred to a specific missile installation. However there are many unresolved historical issues surrounding the downing of Powers' flight and whether he was simply shot down as reported or whether some type of espionage was involved. In any case, the goal of Alsop's article was to score PR points against Eisenhower by putting him in a position where he could not refute an allegation without revealing classified information, for which the NYT can be blamed independently of whether the Soviets already had information about the U-2, IMO.
"However there are many unresolved historical issues surrounding the downing of Powers' flight and whether he was simply shot down as reported or whether some type of espionage was involved."
All accounts we have indicate that the Russians threw EVERYTHING they could at Powers' U-2, and managed to bag several of their own MiGs in the process. We sent Powers on a last grand tour of Russia's most heavily defended sites, on a mission that would have spent 12 hours over hostile territory (flight plan was Pakistan to Norway), so the risk level was far higher than it ever had been. Espionage may have played a role--but the level of risk was frightfully high, even if the Russians had no idea it was coming. Again, State was getting detailed complaints from the Russians (time of flight, altitude, and locations overflown) from the first mission. Lockheed spent considerable time and energy on trying to make the Dragon lady stealthy--they found they could, but only by slowing it down and lowering its airspeed.