As per the conclusions of that great video, going back before Pong and defining a "first" video game depends heavily on your definition of both "video" and "game"

See also Wikipedia's overview: https://en.wikipedia.org/wiki/Early_history_of_video_games

If you want Tennis for Two (1958) to be first, you have to introduce criteria that excludes OXO (1952), Checkers (1952), and Sheep and Gates (1952)

I don't think it is unreasonable to define a "video game" as one employing video graphics and real time input. Things like Tennis for Two (and the later Spacewar) are clearly video games in a sense that mere simulations of board games are not.