Yeah, airplay and Chromecast are much more device agnostic in that respect, maybe because they’re not associated with an account. An Alexa account largely “owns” echo and Alexa enabled devices and also associated music subscriptions (one of each kind per Alexa account e.g. Amazon Music, Spotify etc, but not multiples of the same kind), and only devices in that account can use its subscriptions and vice versa. To be honest, an Alexa account really lacks proper profiles that can own or link to their own resources - it’s much the same with smart home devices where, by and large, only echos in the main Alexa account can directly control them, even in an Amazon household (which is a linked part of adult accounts).