I see on the Last.fm/Deezer FAQ: “supports scrobbling internally in almost all apps independent from the device” -- Emphasis mine. Is Alexa the exception to that (the reason why “almost” was used)?

Hardware implementations of the Deezer app often do not include their internal scrobbling option unfortunately, maybe for trimming the system resources of those apps or simply because the developers do not know what it is. As far as I know, smart TVs do not scrobble either, not sure about game consoles, network streamers and wireless speakers. So it rather is an exception that Chromecast scrobbling with Deezer seems to work, as you mentioned.