"Mayo’s LLM split the summaries it generated into individual facts, then matched those back to source documents. A second LLM then scored how well the facts aligned with those sources, specifically if there was a causal relationship between the two."

It doesn't sound novel from the article. I built something similar over a year ago. Here's a related example from Langchain "How to get a RAG application to add citations" https://python.langchain.com/docs/how_to/qa_citations/