The First Purge is a prequel film and the first instance in the film’s internal universe where the annual murder-fest takes place.
This is interesting reasoning and it actually makes a great deal of logical sense given the premise of the film.
There isn’t a precedent for this event, and thus all of the characters in the film are experiencing the Purge for the first time.
By not watching the previous films before filming, as Rotimi Paul told Pop-Culturist, he took a method approach to experience the Purge for the first time just like his character.