Monroe appears on famous-synesthete lists because of a passage in Norman Mailer’s 1973 biography describing her as having a displacement of the senses, sometimes glossed by later writers as synesthesia. That is the trail: a biographer’s literary characterization, written after her death, with no known statement from Monroe herself and no specifics about what she supposedly experienced.
By the standards applied everywhere else on this list, that is not evidence of synesthesia; it is evidence of a sentence in a biography.
We include her because the claim is common enough that people search it, and the useful answer is the honest one: unverified, secondhand, and probably unknowable. Lists that state it as fact are copying each other, not a source.