Where does it say how the actual phishing message itself is easy to send? I see no explanation there. How does one send a message with a different SenderID?
I didn't want to explain that in the article because I don't think it adds value.
I suggest you read up on the SMS protocol, basically anyone on the network, if able to inject any arbitrary packet into it, can.
If you don't even want to buy equipment or even code, take a look at AWS SNS or (i believe) Twilio too.