Sure self hosted matrix is a lot safer than sending your messages through meta servers for example.

A lot safer in which case? I can imagine a few very real ones where it’s not.

Self-hosted Signal (requires patching the client, but it’s straightforward) server I would understand.

but for critical government applications and especially the military it might be. That is what we are talking about here.

Signal devs have a few papers describing how and by what logic they are addressing these problems.

Again, self-hosting (because accounts can be blocked by Signal) their solution is a better idea.