Sorry, he didn’t knock her out, but what he did was assault. If you swing and miss someone, that is also assault. Agree, in a real world situation this is nothing, but it is assault by legal definition.
From https://www.criminaldefenselawyer.com/resources/district-columbia-assault-laws.htm
In the District of Columbia, it is a misdemeanor to:
assault another person, or threaten someone in a menacing manner.
(D.C. Code § 22-404.)
Under general legal principles, an assault is a physical act or a show of force that puts another person in fear of imminent harm or offensive physical contact. For example, hitting someone with a fist could be considered an assault, and so could throwing a beer bottle at an individual. Because the Districts laws criminalize both assaults and threats, neither injury to nor physical contact with the victim is necessary to violate Washington, D.C.s assault law.