Is each antenna directional, or do routers only gain directionality with multiple antennas? I'm amazed that they can get such precise beamforming with only four

Is each antenna directional, or do routers only gain directionality with multiple antennas? I'm amazed that they can get such precise beamforming with only four or six antennas (maybe they didn't even have that many antennas in 2013.) Admittedly I'm more familiar with beamforming as a sonar concept than as radar. edit: I just read the MIT paper, and it's using ISAR. My advisor would be ashamed of me for not realizing that... Still, as the number of antennas continue to increase for better directionality, the wifi radar could even become effective on stationary targets. Soon we'll all be dressed like F-35s to hide from our routers.