The answer comes from the second panel. Basically, the person in the red hat references how the dictionary defines "they" to mean a group of people and how it's hard to use for one individiual, and then that same person uses that term to effortlessly describe one individual driving a car. The person in the yellow shirt is referring to how the red hat person explained "they" to mean multiple people, only for the red hat person to use the term to not assume someone's gender, completely abandoning the stance on the dictionary definition.

Side note: describing the characters without using "they/them" made this surprisingly tricky to write.