Matrix leaks tons of metadata, and its encryption lacks perfect forward secrecy. Additionally, it requires an email to sign up, and there are accounts with unique identifiers.
Simplex does not have any accounts or identifiers, everything is stored entirely locally. Additionally, it is based on the double ratchet Signal protocol, with improvements made for post-quantum encryption. It does not require anything to sign up, as there are no accounts. Metadata is not leaked as it is with Matrix, as everything is encrypted or obscured. Messages are padded to 16KB, the sender/receiver is not attached to the message, and there are fake messages being sent to obscure the identity and frequency of contact of those you are talking to even under monitoring of your network. Additionally, for anonymity, SimpleX is allowing for repudiation so that you cannot prove that a specific person sent specific messages, allowing doubt if messages were to be use in a court case, for instance. It is the trend (especially from a security perspective) to implement nonrepudiation, but the SimpleX team decided to remove it to protect users (after years of it being present in SimpleX chat). This is a protection intended for journalists, but it extends to many other cases as well.
Matrix is a nice toy, but SimpleX chat is built for anonymity above all else, and it does that job far better than Matrix ever has or will.