As far as I could find, the text suggests that only applies to market giants (i.e. Meta with WhatsApp and Facebook Messenger). iMessage is not even breaking the top 5 in the EU. It's not even in the top 5 world wide, and even in the US it's only somewhere in position 4 or 5. If they go for absolute numbers (i.e. X million users) instead of market share that might be different, but it's unlikely to really be relevant to the EU considering from a user's perspective it's already interoperable (messages sent on one device end up on the other device, even if it's technically a mix of SMS, MMS and iMessage - that part is not really relevant).

I think the only source of all this iMessage this that and the other comes from parts of American society where they value the color of a chat bubble. Ironically that value has nothing to do with iMessage and just to do with "this persion I am chatting with can afford an iPhone", which in turn is what people appear to value.

In countries where iMessage is not really used, it doesn't matter at all. I would be surprised if most users would even know about different chat bubble colours and what they mean.

It's already been made public that iMessage will not (currently) be covered under this.

iMessage is pretty much dead in the EU though so I doubt the EU regulators will ask for interoperability.