That’s a very interesting interpretation of what you’ve quoted.
This is not stating that the lowest population realms we currently have are equivalent to the highest population realms in 2006. It’s rather stating that a full realm in 2006 would be considered a low population realm by our current metrics.
If we contrast that with ironforge.pro
for example, we can see that the highest population realms have more than 10,000 weekly logs for unique raiding characters. Servers with around 2000-3000 weekly logs for unique raiding characters are considered Low by our current metrics, but may have been considered Full in 2006.