What’s being portrayed is broadly plausible for late-19th-century photography, but likely dramatized.

For large cameras of that era, the lens cap could indeed serve as a simple shutter for relatively long exposures. Subjects were often told to hold still because exposures were not instantaneous by modern standards.

Flash powder (typically magnesium-based) was also historically real and burned very brightly. In an outdoor daylight group portrait, though, it probably would not be the sole exposure source. More likely, it would act as fill light to brighten shadowed faces, especially under hats, while daylight provided most of the exposure.

The movie likely understates the brightness and visual impact of the flash; real magnesium flash was intense. The staging also looks cinematic rather than ideal for an actual formal group portrait: a photographer would typically place the camera more squarely in front of the group.

So the scene is best read as: lens cap/open exposure for the plate, with a flash added during that exposure to boost the shadows. That said, the exact timing and effect were probably adjusted for dramatic effect rather than strict technical accuracy.