Everything is possible in Brazilian Jiu-jitsu. Although this is not something we can see every day but it’s possible. This happened in the NAGA tournament in Germany in 2015.
We’re not sure if the white belt is training in some other grappling sport but he showed up a very good performance against a Black Belt and managed to submit him. Check out the video below how he did it.
https://bjj-world.com/female-bjj-white-belt-defeats-male-bjj-black-belt/