Do you not consider Alpine Linux to fall into the general category of “Linux”, then? It lacks GNU user space utilities, though there is never a world where I would not consider it a “Linux” operating system. You seem to be overgeneralizing here and making assumptions about OP’s intentions that aren’t based in fact. I don’t see the point in drawing meaningless lines, here. What you’re referring to (as described by the GNU project) is GNU/Linux, not “Linux” by itself. The two are often but not always used interchangeably, and treating them as exactly the same leads to major outliers, like Alpine. I’ve heard plenty of people use the term “Linux” in practice to describe software running on embedded devices that don’t contain GNU utilities, so this isn’t exclusive to Alpine. In fact, the only real exception that I see consistently to operating systems that run the Linux kernel is Android, so it makes much more sense to formulate a description of the generic term “Linux” as simply having an exception for Android, though I’d argue that the only reasons that Android isn’t viewed as “Linux” is because it is a mobile operating system, it is developed with the sole intention of including non-free, proprietary software (AOSP by itself isn’t meant to be the full operating system on any device, but rather a framework), and the fact that the structure of the filesystem and the way apps are run differ completely from the ways of traditional “Linux”. It seems to be an exception purely by the fact that it operates in fundamentally different ways than other “Linux” operating systems.
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.