Amazon might have been the first to launch a dedicated device that can do voice search but Google was doing it(on Android) before such a device was launched by amazon.
The context in the grandparent was Alexa (i.e. voice search at home) which is much much narrower than voice search in general.
Here's an excerpt of Jeff Bezos' statement regarding doubling down on Alexa so its clearer this is not about voice search in general or on smartphones but on their smart home devices using Alexa:
“We’ve reached an important point where other companies and developers are accelerating adoption of Alexa. There are now over 30,000 skills from outside developers, customers can control more than 4,000 smart home devices from 1,200 unique brands with Alexa, and we’re seeing strong response to our new far-field voice kit for manufacturers. Much more to come and a huge thank you to our customers and partners,"
https://mashable.com/2018/02/01/amazon-earnings-2017-alexa-j...
Long before voice assistants/search became a thing with smart phones there was Google 411 ... which no one remembers.
I'd guess that 20% of Google's mobile queries in 2016 are a few orders of magnitude larger than Amazon's total voice search queries, effectively making Amazon's voice queries a drop in the ocean.