The article uses confusing terminology. There are three classes of rockets (in the USA). The lowest is "model rocketry". For this you can build any airframe you want, but you can only use off the shelf engines up to size 'G'. Then there is high power rocketry. For that you can again make your own airframe and you can use engines up to class O. These first two classes must use *premade* motors. Though there are provisions in High Power for engines with consumable parts and durable parts--so the whole thing doesn't need to be replaced. The last class is amateur rocketry. For that, the sky's the limit. You need DOT explosives permits and various other certifications.
So, this article seems confusing when they say 'model' when this is likely 'high power' rocketry. Maybe even amateur rocketry, but I doubt that any intern there will have the permits. Maybe one of the full time staff did that part.