Vector Retrieval Quietly Replaced Keyword Match, and the SEO Stack Did Not Notice

How dense embedding retrieval replaced BM25 in modern AI search, what the mechanism actually does, and why exact-match SEO tactics quietly stopped working. There is a page I audited last year that ranks well gets cited, gets quoted, gets used as a source by AI assistants for a phrase nobody types. The literal string appears nowhere in the document. The document is about the topic, plainly and accurately, in clear prose. The query is a paraphrase. Twenty years of SEO heuristics would predict this page does not match.