can diffs be piped through an llm to give you something higher level but still tie it back to to changes

You can, but it's slow, expensive, and hallucinates. An LLM looking at a raw diff might miss a renamed function or invent a dependency that doesn't exist. sem does it structurally: parses both sides with tree-sitter, computes structural hashes, walks the real dependency graph. If you want to layer an LLM on top for summarization, you're feeding it 10 entities instead of 500 lines of unified diff.