Mastodon doesn't actually have communities built in, so if you want to post in a Lemmy community you have to @ the community. (That means that there's a decent chance this post was an accident, since linking to a community and posting to a community is the same on mastodon)
Microsoft was claiming that the data would be inaccessible to hackers (which is not true).
Signal claimed the exact opposite: that once it's on your computer, messages can be seen by malicious programs on your computer.
Signal was caught having less than ideal security. Microsoft was caught lying.