I think this is strictly true. Eriksson says specifically that deliberate practice is only possible when there is a body of training knowledge and trainers. However, also, he talks about how to approximate this in other fields. He suggests the "Benjamin Franklin Method"[1] as one idea and I think it could be used with small open source projects.

Also, projects like exercism.io use other learners as the trainers. I think Eriksson calls this the "top gun" method, based on the flight school and movie.

[1] https://excellence-in-literature.com/copywork-how-benjamin-f...