I was pointing out the same thing. You can only prove that the content-carrier (event in nostr) originates from the owner of the keypair, and even then we assume it is only owned by one person.

nostr:nevent1qqs839hagpp65e6tf82ppw0vk7pd89s6zl0gstma528dqaq34qdet8qpzpmhxue69uhkummnw3ezumt0d5hsygrs0yvdud8zguendmkep65y0uvak363kut4mzfkgf5y56427nvuecpsgqqqqqqs6r36w3