The linked comment doesn't seem to have any bearing on how long Chromecast Audio is likely to be supported?

Regardless of whether kuschku's accusations are true (there doesn't seem to be much evidence, and even if it were so I'm not sure why Google should be expected to assist Amazon in reverse engineering Chromecast), unlike an Amazon clone the Chromecast Audio is a first-party product. Google may have stopped producing the standalone dongle, but the tech is still used by Google Home and other speakers with integrated Google Cast support, so it's not disappearing anytime soon.