Let's say you have 100,000 posts, and 5,000 members... the import will be speedy. But if you're importing a 1,000,000 posts site, and with 10,000 members in the database... That means you'll have to wait a long time to have it imported.
But this is proportioned to how much horsepower your server has, which makes the process a little better.
The import does big boards without issue as Brogan said, however... the quote you posted is old, was more talking about speed differences and limitations at the time.
For example, back then, we couldn't import a vB4 database. And even if there was a plugin for it, there was a few, but minor limitations to it because of how xenForo worked at the time.