One of the new devices the "Echo Plus" does reportedly contain a "Smart Home Hub" and contains a zigbee radio in it
From their page:
https://www.amazon.com/dp/B075RWFCHB
Alexa can control your compatible smart lights, thermostats, locks, garage doors, sprinklers, and more. Set up devices from Philips Hue, GE, Kwikset, and others without the need for a separate hub. Just ask Alexa to discover your connected devices.
Right now they are a compliment, they have the budget, user base, and desire to be tightly integrated to the home. I don't think they are a competitor yet, but I think of the whole list of c4 killers above - Amazon has the most potential. If nothing else they have the largest ecosystem of all the other platforms.
All the other platforms boast about being the smarthome platform of the future. Amazon is quietly doing it in a non-threatening way.