Just to clarify: Google supports two kinds of messages. Broadcast, which in nature is to all speakers and does not interrupt a music stream, and Text to speech, which is basically casting an audio stream. The last one stops playback of other streams ad does not resume them. The solution above adds broadcast support to Home Assistant, which without this solution only supports text to speech.
For those who want to target specific speakers there is another solution on the forum. It does not require docker but uses scripts.The benefit is a) resume playback after text to speech on a specific speaker and b) the possibility to know which unit received a speech command so you can direct an answer right back (in combination with other scripts). That solution looks rather complex with scripting though. So both solutions have their merits.