It definitely is a standard, defined by 3GPP. Apple alone doesn't get to decide what is a standard. Google's Messaging app is just one implementation of the pr

It definitely is a standard, defined by 3GPP. Apple alone doesn't get to decide what is a standard. Google's Messaging app is just one implementation of the protocol. There are other implementations, that are actually used. Samsung have their own, some carriers have their own. It really is an ecosystem.