It doesn’t have to be open, just provide publicly accessible APIs so that apps can interconnect with it. Google provides this, Apple does not.
To be clear IDGAF about Google. I promote RCS and you can say it’s not a standard, but it is. It’s maintained by the GSM Association and they put out a universal profile that anyone can implement and extend just like Google did and Apple could easily do. They’re just extending an existing standard.
Even in the Google messages app I can change the RCS backend servers at any time, you don’t have to use Googles RCS implementation