I did a test and Catt seems not capable of casting to speaker group. It plays on the single device port.
And i’m using catt to cast files and sites. In the older version i used the castv2-client lib, thats capable of address the different cast ports.
In the latest version is use castv2-client again for casting text. So play text should work on speaker groups.
I will investigate if going back to castv2-client is a better option.
For the short term you could use the GroupDevice in Sounds. The difference with the google speaker group function is that the playing on the different devices is started sequential.