Imagine human capability as a high dimensional blob.^1 The knowledge frontier is the surface of this blob.
I imagine the edge of human knowledge and capability (the Knowledge frontier) as an ever-expanding surface — everything from theories about the cosmic microwave background and the beginning of the universe to the ability to turn bauxite into aluminum via the Bayer Process and the Hall—Héroult process.
Frontier Technology is technology that sits on the surface of the knowledge frontier.
The Knowledge Frontier is agnostic of the environment and has nothing to do with impact.
It might be that the The tech tree is a scaffolding inside the knowledge frontier!
^1: According to a mathematician friend, the correct term for this is a “compact” but for the sake of evocative language I’m going to stick with “blob”