You’re absolutely right. I’ve tried multiple times by adding random delays when sending messages via AppleScript on my MacBook, but it doesn’t change anything: the account gets shadowbanned or restricted after just a few messages.
Some people in China are said to use more advanced methods that they only share with a very small group.
However, according to the media, they claim their approach is cheaper than SMS, even though they still need to use large iPhone farms and a huge number of SIM cards to send messages.
In short, their methods for sending phishing messages via iMessage are quite closed off and not easily accessible.