It's not even clear what Google wants Apple to do to remedy their complaint. So you sign up for iMessage Business and you should therefore be able to send "enriched" messages to more than just iOS users. Using what?
I believe it's about integration with RCS. Now, I don't know much about rcs but presumably it's similar in features to imessage. So Google wants Apple to adopt RCS so you can send "blue" messages to others outside Apple's network.