You're right it's not the media that matters, it's the message. But the reality is that people are being arrested for unambiguously political messages [1]. Nobody would care about people being arrested for making credible death threats over twitter, versus over snail mail. When people say "arrested for tweets" they're referring to stuff like the above link.
1. https://www1.cbn.com/cbnnews/world/2022/august/army-vet-alle...