Part of that engineering focus is an insane attention to the BOM, and to support(call center) and warranty (returns & repairs) costs. I think both of these things make it unlikely that Garmin will add a cellular modem to start tracking otherwise unconnected devices.
You can push activities to your Garmin account but they don't bother engineering in any additional functionality, eg, to show your last activity if uploaded from outside, or to account for externally uploaded activities in your training metrics. It never feels like they're interested in improving the functionality of existing devices.
Don't get me wrong, I'm happier with my Forerunner 945 than I was with my Apple Watch, but only just. Basically, it does enough things that my Apple Watch can't do, to make suffering with the majority of things it's somewhat worse at, worthwhile as a tradeoff.
Do they develop their own ASICs?