Apparently the new app - bringing with it the long requested message transport unification - will be compatible with other Garmin inReach devices (following a firmware update).
So which devices???
On the one want I can't see any technical reason why this shouldn't be ALL of them (the new special sauce all happens on the phone, the device itself needs to know nothing about it).. on the other hand I can see Garmin possibly using this as a nudge to encourage users of legacy products to upgrade, especially as I suspect inReach products likely have a longer owner-decides-the-need-an-upgrade cycle vs other Garmin devices. Then again, inReach product lines stand out as one of their few lines to attract ongoing monthly subscription (vs. upfront cost only) so it does still pay for Garmin to keep their existing user base happy with new features.
(said differently: I really hope they give their legacy products some love here - I really want this feature!)