They run these clients themselves and the redis instance isn't publically exposed.
It would indeed be very strange to hope your random users coordinate with your client side load balancer. You wouldn't even have to send real traffic. You could just manipulate redis directly to force all the real traffic to go to a single node. DoSing redis itself is also pretty easy.
I don't think the article implied that the client was for some sort of internal server-to-server communication, or that the Redis instance was directly exposed to the internet.
So no, I don't think they run these clients themselves. If the code runs out there, it's open to inspection.
Either way, you are right to point out that it important to only a try a pattern like this if your clients are highly trusted (or/and have additional compensating controls against DDOS threats). It would be beneficial if the OP made more explicit what their client/server relationships and also flagged the risk you mentioned for general audiences not to go implementing such a solution in the wrong places.
It would indeed be very strange to hope your random users coordinate with your client side load balancer. You wouldn't even have to send real traffic. You could just manipulate redis directly to force all the real traffic to go to a single node. DoSing redis itself is also pretty easy.