At Comic-Con earlier this year, Marvel made it clear that Black Widow would be set between Captain America: Civil War and Avengers: Infinity War, giving the idea of this cameo some credence.
What I like about this alleged info is that it would finally pinpoint more specifically when Black Widow is set.
Marvel has brilliantly started playing around with timelines, giving us stories that occurred earlier in the history of the MCU with Captain Marvel.
While not confirmed, in some ways this would actually make a lot of sense.