The proposer of a governance action is whoever submitted it to the chain. Submitting an action requires locking a deposit, and the action names a reward (return) address that receives that deposit back once the action leaves the voting process; that address is the on-chain trace of who proposed it.
The chain itself does not record a proposer name. DRepTalk identifies the organizations behind well-known return addresses through a curated list, and falls back to an identicon derived from the address for everyone else, so two actions from the same submitter are recognizably from the same source.