By shady guy, the commenter is likey referring to peer-to-peer marketplace swapping of fiat and crypto, not DEXes. DEXes cannot offer fiat currencies since it is inherently centralized. AFAIK, the only options to exchange fiat are centralized exchanges and peer-to-peer marketplaces, both of which rely on extensive trust in a very small number of people, and are not open-source or publicly-auditable.