I can't tell from Mr Baum's article where he got his frustratingly incomplete info. But, though filled with typos, the event information given at EventBrite seems crystal clear to me. After describing the market as a sprawling event with free areas (crafts and food trucks) and paid areas (with bars and entertainment 21+), with an all-day "happy hour" deal for $25 that includes $20 in drink tokens plus other goodies, it goes on to describe the outdoor screening in the 21+ area (SLU Discovery Center):

Outdoor Cinema | 21+ Only | 8pm -10pm

The outdoor cinema screening will start promptly at dusk which is closer to 9-9:30pm with seating opening at 7pm on the grass lawn. It will be first come, first serve with no seating provided. Attendees are welcome to bring blanket and picnic, but no outside alcohol, so bag check will be enforced.

We will begin seating folks at 8pm, so arrive early for good placement. Music by DJ will be provided in the duration prior to the movie, along with food trucks and shopping outside the 21+ area.

While one could quibble that on the night of the event "promptly at dusk" should be an easily defined time, and not sometime in a half-hour range, the rest of the info is quite clear. Perhaps at press time Mr Baum just hadn't had his coffee, and was feeling rushed.