It's not about lumens. Its the beam pattern and lux.

It's the same as a high or low beam on a car. A regular flashlight is equal to a high beam. A proper bike light has a cut of to act like a low beam.

Put the bike in front of your car with the light on and sit in the car to see what the effect is.

The Germans even made a law regarding bike light needing to be a low beam; the StVZO standard. It's just the Germans that made it in to a law, but it doesn't beam blinding people else where is good.

Random google hit on StVZO: https://www.bikeradar.com/advice/buyers-guides/stvzo-bike-lights/

A regular flashlight beam battern is however fine for mountain biking; as you don't have other traffic and allows you to see branches. Although i would recommend to have a headlight and handle bar mounted one, so you can see in to the corners.