They might be using a hosting that doesn’t charge insane prices for egress.
Both Hetzner and DigitalOcean give first 20-23 TB of egress per month for free and then charge $10/TB overage.
If magecdn doesn't support setting a cap then their "no surprise charges" is misleading. But if they do support caps then it should be mentioned in the FAQ imho, it's a selling point.
>All root servers have a dedicated 1 GBit uplink by default and with it unlimited traffic. Inclusive monthly traffic for servers with 10G uplink is 20TB. There is no bandwidth limitation. We will charge € 1/TB for overusage