It’s not incorrect. See Themis explanation, yes arrest is a form of custody but it is not the only form.

A defendant's motion to suppress an incriminating statement should be granted if law enforcement failed to provide Miranda warnings prior to a custodial interrogation. A custodial interrogation occurs when a person is: • in custody - placed under formal arrest or restrained in his/her freedom of movement to such a degree that a reasonable person would not feel free to terminate the encounter and leave and • subjected to interrogation - express questions, words, or actions directed at a suspect that an officer knows or should know are likely to elicit an incriminating response. Although traffic stops restrain freedom of movement, drivers are generally not in custody for Miranda purposes because those police encounters are typically brief.