The way I read this is: 1. iOS and macOS allows you to type out an SMS message on your mac, which is delivered via the same mechanisms and channels as regular i

The way I read this is: 1. iOS and macOS allows you to type out an SMS message on your mac, which is delivered via the same mechanisms and channels as regular iMessages to your phone, causing it to relay and send out the SMS on behalf of your mac; 2. it turns out anyone on the internet can send you an iMessage formatted as if it was an SMS relay request sent by your mac to your phone, and your phone will relay SMS out to a third (fourth?) party - and nothing checks to see if the relay request actually came from your mac.