I work for a large CDN. Lots of big customers do this, they shift their traffic around between different CDNs based on a lot of metrics, including price and performance.
Some of our stuff dynamically utilizes other CDNs. A customer might go try and test deploy something and we might use (AWS or whatever) to do it.
It's CDNs all the way down.
We should rename our product "Infinite Regress" ;-)
We the whole stack for most people, but also run our software on "private cdns" for some. We primarily target devs: https://fly.io