The important thing is that all the code written by Core Devices is open source. They can't force third parties to open their code, but they're opening all of their own work. And that proprietary code is not required to use the watches. Most of them don't even have heart rate sensors, and clearly Memfault is not required. They're committing to maintaining a 100% open source version that still allows you to use the watches with minimal compromise.
Then say "Pebble Watch Software written by Core Devices Is Now 100% Open Source", or "Pebble mobile app open sourced" (that seems to be the major actual change?), or something like that. The thing they've actually done should be commended, but that doesn't mean ignoring that they've chosen to make a claim in the headline that isn't actually true.