Thanks for confirming the site, I was looking at the wrong one.
As far as I know, that line is not from Ultra. What you’re seeing there is DNS prefetching, I’d guess that’s being done by your performance plugin.
DNS prefetching is an attempt to resolve domain names before a user tries to follow a link. This is done using the computer’s normal DNS resolution mechanism.
Put another way, this is intentionally being inserted by a plugin. The link checker is incorrectly picking this up, it’s a false negative.
(A little more on DNS prefetching.)