The simple fact that they are former employees is meaningless. This is especially true in California (i.e. where Twitter HQ is, and presumably most of these employees) where non-competes are nearly completely unenforceable. Twitter will have to specifically show that it's about their internal trade secrets, and not just the general experience they brought from their time at Twitter.

But right now, it's entirely Twitter doing the talking. We haven't seen yet how Meta will respond. I predict there is a 0% chance that Threads gets shutdown any time soon.

If you read the actual letter, it seems to paint a slightly different picture. They vaguely order Meta to stop using twitters trade secrets (whatever that may be), and serve notice to preserve communications. That's fairly normal. But then they have an entire tangent about scraping Twitter's publicly available data.