What this Act says is that Apple Messages must interop with any other messaging system. If you spin up a VM running an iMessage-compatible server on Hetzner, Apple Messages must interop with you and cannot privilege Messages (e.g. by continuing to use blue bubbles as a differentiator). That VM may be malicious (for ex: it may log who communicated with whom when), but the Act still requires that it be placed on an even footing with Messages.
Similarly, the Act essentially says that once implemented, anyone using a phone can unintentionally be sending all of their messages to Facebook Messenger, which by law must have seamless interop with Messages. Any group chat could be logged by Facebook by virtue of one person in the chat choosing Facebook Messenger as their default messaging app.
This requirement materially changes the security posture of a billion devices currently in use. You may believe the tradeoff is worthwhile, but it's still not a free tradeoff.