A little-known fact about the echo smart speakers is that they run a minimally modified version of FireOS at their core. In fact, many references to the Fire tablet lineup can be seen around the echo firmware. Using the same SoC makes sense in this regard as it allows for a more unified build process by enabling kernel/driver/firmware blob reuse.
There are some sparse details about the original Echo floating around[0], but I couldn't find the specifics.
While they may be able to share some firmware, I'd still wager their kernels/drivers/firmware blobs are different enough due to drastically different components interfacing with the SoC. I personally question the value of putting Android on non-mobile screen-less devices (forgetting about the Echo Show).
0: https://labs.mwrinfosecurity.com/blog/alexa-are-you-listenin...
While reuse is nice, optimizing for experimentation and different products is probably more useful. Given most of the magic of Alexa lives in Amazon's servers, there seems to be less incentive to use existing solutions over what makes sense (and is cost effective).