That seems reasonable. Customers can order directly through the device ("Alexa, order some toilet paper"), that's low friction and lets Amazon rank options to their benefit. Plus regularly using Alexa keeps Amazon primed (no pun intended) in the customer's mind so next time they go online shopping they default to Amazon. Once you're using Alexa, there's an Apple effect encouraging adoption of other Amazon products through ease of use (Amazon Music, FireTV, Audible, etc). Amazon is also monetizing their products with ads, I'm sure that's part of the Alexa strategy as well.

Remains to be seen whether there is a "pull back" from smart devices that impacts the play. I was an early adopter of Alexa... at this point no one in my technical circle has proprietary "smart" home automation devices anymore, and the non-tech folks at best use Alexa to search in the Amazon app (mainly because they struggle with phone keyboards)