Yes. List it. You can consider that experience from the very beginning.
Open source contributions are valuable and if that’s the only source of their experience coding then it’s the best they can put on their CV. I don’t know why people is saying otherwise but, as a recruiter myself, when evaluating a candidate it will be an extremely valuable information.
Will I consider the candidate a senior or mid level developer if he only has open source experience? Unless he’s the main maintainer of a sizeable project, probably not.
Would I consider it a very good sign for someone looking for their first job in development? Absolutely. At the very least it indicates a very important quality, a liking for code and a desire to improve and learn. Believe me, nothing gets better than that for juniors.
The point is when you have no other relevant experience then that needs to go in. If you had other significant experience that just nerfs the time spent on open source, then I will still put it, but maybe in an “other” category.
But I can assure you, when I look at CVs i actively look for those things and, in juniors, I consider it a good sign and a much better sign than a CV without anything on the “experience” part.
Just make sure the information there is honest and that, during the interview, the experience is put forward without exaggeration and it ca work very well.