I know ppfd is more important for plants than lumens, but I think considering we're talking about the sun and its light, we can somewhat use it to mentally see it affects flowering (or doesn't).
Finally, to put things into perspective, it's worth noting that a full moon on a clear night results in the Earth being illuminated by about a quarter of 1 lux at the Earth's surface. In comparison a typical living room will have an illumination of about 50 lux and direct sunlight results in anything up to about 130,000 lux at the Earth's surface.