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My experience is that it rarely matters — you can try to define precisely what is or isn't IoT, but you're probably wasting your time splitting hairs rather than solving problems.
For a good overview of what different groups consider "IoT", you might want to read What classifies a device as IoT? — you quickly see that one question gives you at least ten ...
You're right, there is a grey area (even if there are some formal definitions, it is still maybe early to be definitive).
One key point about some wearables (in contrast to a smartphone) is that they rely on a 2nd device (such as a phone) to enable features. Both for configuration, and as a proxy to connect to the internet. It is this machine-to-machine ...
For the most part IoT is a buzzword since all the "Things" (including PCs, Tablets, and Smartphones) use the same internet. The biggest difference is the perception to the user.
10 years ago a smart phone would have been considered an IoT device but today, since almost everyone has one, it is just a phone. If it is a physical device, connects to something ...
In about a years time you can get an nRF91 + nRF52840 combo. LTE(CATM1 and NbIoT), GPS, BLE, USB, 802.15.4, etc, that uses a fraction of the power of current devices, and got plenty of µC-resources to spare.
Haven't tried it myself, but a few things to consider:
If you had the two devices with line of sight to each other in a very very large open space, then there should be a direct relationship between RSSI and distance. However that is nearly never the case. With obstacles, reflections, multipath, and more, there's significant variation in the observed RSSI.