You left out an important part of the quote:
- Requires a SmartThings Hub or a compatible device with SmartThings Hub functionality.
Those compatible devices with hub functionality are plug in sticks which can be added to some models of television sets or the Nvidia shield. They have the same radios inside that a smartthings hub does. But to be honest, I’m not sure those are even still sold.
As @nathancu mentioned, all you need for what you want to do is a sensor, any brand, that can trigger an Alexa routine.
Some zigbee sensors can communicate directly with an Echo Plus or an Echo Show 10” 2nd generation, The echo models that specifically say they have a “zigbee hub inside“.
If you don’t have one of those echo models, there has to be some way for the sensor to communicate with your Alexa account. Most will require a hub, but there are a few Wi-Fi sensors which do not.
Since right now all you have is an echo spot, I suggest you ask your question in the Amazon help forum and people there should be able to suggest sensors for you which will work without a hub: