That's the real issue - previously these DNS servers did not work well with CDNs because they didn't send the location of the client making the request to the origin server. This extension fixes that problem.
If you aren't using these DNS servers then CDNs probably route your requests properly.
I am using Google's, but that isn't really the point; the point is that in real-world usage, the example in the article of a multi-Mbit connection ending up with Kbit-level throughput due to being routed to the wrong CDN is very unlikely if you live in the U.S. or Europe. It would require either a really broken TCP stack, or gigantic satellite-internet-level latencies for latency to constrain throughput that much.
Yes, you are probably right about the throughput. The latency problems are very valid though, and make websites seem remarkably slower.
I live in Australia at the end of a very long trans-pac pipe, and properly configured CDNs make a huge difference. A great example is how a few cheap ISPs here route based on price, not latency. That meant that when Amazon opened their Singapore dataceter a visitor from Australia (using one of these ISPs) could be routed via the US West Coast.
That's the real issue - previously these DNS servers did not work well with CDNs because they didn't send the location of the client making the request to the origin server. This extension fixes that problem.
If you aren't using these DNS servers then CDNs probably route your requests properly.