to send a message (or call) to a phone number the sender of the message has to pay the company that owns the recipient number a fee. the fee varies. in the u. s. it’s very small. less than a penny. in some undeveloped countries it’s much higher. most fraud like this occurs in those countries with higher fees

let’s say i make friends with a mobile network operator in a country with high fees. i tell that mobile network i can send a lot of traffic to your network and generate you revenue in message passing fees. but i need to get a share of the money. the mobile network operator understands my methods are not legit and will not block the traffic / apply normal spam / anti fraud protections. we both make money at the expense of the message sender

I didn't fully understand it either, but my take is this:

1. Twilio (or someone else) sends a text message for OTP.

2. This message needs to pass through my carrier (MNO, in the article) for delivery, and my carrier charges Twilio and/or their carrier, for the service. This is the revenue mentioned.

3. I have an agreement with my carrier, or simply form my own, that we split revenue.

This is a correct understanding

The key here is that sometimes receiving a SMS costs money.

So the attacker creates the fake traffic for the operator, operator generates revenue and then pays the attacker.