> 500kB packed array, coupled with cooldown logic, should have a negligible performance cost and can easily be done on a single thread. That thread could interact via atomic channels with the CDN
That's not simpler.
We used tools that we're already using heavily in production and are comfortable with.
With respect to your experience in the matter, I strongly disagree. What I described is complicated to say, easy to implement. What the OP describes ("use redis") is easy to say, complicated to implement. Not just in terms of human work time (setting up the redis machine and instance, connecting everything together), but also in terms of number of moving parts (more machines, more programs, etc.).
> We used tools that we're already using heavily in production and are comfortable with.
That's entirely fair, and what I figured was the most likely explanation.