I think you're confusing the two benefits.

What the thread OP is talking about is the decrease in page size from the URLs included in the page source. Choosing a shorter URL for the static domain versus a longer one resulted in a rough savings of 50 bytes per page (size), compressed.

You're referring to request cookie size, which was also decreased significantly (CloudFlare still sets a single cookie, unfortunately), which results in the big savings of ~100 KB upstream per page load.