When a founder says "You might be surprised to learn there’s no Faiss inside Pinecone" I'm inclined to believe them. They seem like a good team. I don't think they would lie about this.
I also just had a browse through some of the employee profiles on LinkedIn and their team absolutely seems to have the background you would need to build something like this from scratch. https://www.linkedin.com/company/pinecone-io/people/