It’s more than theory, it’s backed by solid evidence. Suzanne Simard has shown not just communication but resource sharing, preferential treatment for family members, inter species cooperation. I don’t recall if she was in that documentary or not. She discovered this starting in 1995, which means there’s a lot left to learn about the implications.

https://mothertreeproject.org/about-mother-trees-in-the-fore...

https://en.m.wikipedia.org/wiki/Suzanne_Simard