No, it’s two separate things.
About stereo pairing: they’re both ASE, but different generations. A bit like the new Beosound A9 and Beosound 2 3rd Gen are small upgrades over the older Mozart platform.
The ASE platform on which M5 and Beosound 1/2 1st Gen are based, doesn’t support stereo pairing in any way.
For what concerns multiroom and groups via Google Home, that’s indeed related to the older Chromecast firmware version (v1.29) for which Google has removed the ability to play in groups (v1.52 is the minimum required firmware now for using a Chromecast built-in speaker in groups).