From what I can make out, the thing shown on the door is the bit that connects to the hub, and is activated by the key thing being within around 2.5 m / 8 ft, or by the keypad. This seems an interesting idea, though only if you can buy extra keys. The retail price being suggested on the sites advertising it is of the order of £110, which seems eye watering for what is really a more secure version of hiding a button behind a bush and hoping no one is watching.
I guess if you use the arrival/presence sensors with a single access point for things like disarming alarms, and can stop laughing at the price, then it is an interesting alternative and indeed a better idea in some respects. It seems to me that it would only indicate arrival though, not presence.
If, however, you use the arrival/presence sensors as a slightly more hit and miss arrival sensor (anywhere from fifty feet away to walking through the door) and as a suggestion of recent presence, then it isn’t in any way a replacement.
I don’t know about anywhere else, but in the UK a couple of years back the starter kits used to retail at about £200 but were often available from Amazon for £100, while the individual price of the hub was about £90. So unsurprisingly a really healthy market developed in cheap brand new hubs and arrival sensors from starter kits that had been purchased just for the outlet and other sensors (even without selling the unwanted bits, £100 was less than the individual prices for these). This market has largely dried up now, largely because of price changes, and also because it is anybodies guess what Samsung actually sell in the UK now (and I include Samsung in that).
The arrival sensors have always been marmite, and a healthy nearly new market was driven by those who couldn’t get on with the things and just wanted to get rid of them. That seems to have largely dried up now, in favour of unrealistic prices based on scarcity, and a much smaller used market.