I still fail to understand the significance. Even if Guinyard did see a police car after the Tippit shooting, how is that significant?
Witnesses can misremember, especially over time, but even if Guinyard observation was correct, what is that supposed to mean?
The implication is that there was a suspicious "police officer" (real or fake) on the scene before the police were known to have been there.
Kenneth Croy was the first officer to arrive (he was coming from downtown and heading through Oak Cliff on his way home after working the parade when he heard the report on the police radio). Croy was there within roughly three minutes of the shooting. There wasn't officially any police car in that alley at this early stage (if ever?).
