If you're already familiar with ultrafilters, then the relative intelligence definition is SO elegant you're like, "Whoah. How can a definition of relative intelligence be that simple?" Of course, if you're not familiar with ultrafilters, then the definition is just as complicated as the definition of ultrafilters, which is quite complicated. So the definition of relative intelligence is like a simple computer program which imports from a complicated library in order to achieve that simplicity.
A definition, but not a concrete and practical way of determining the relative difference between two agents?
Come on, bring this into reality. We're mostly a bunch of coders here. Stop talking about ultrafilters, concepts that are complex and thus render those of us not familiar with them unable to understand what you're saying.
Answer the question.. How can you take two real agents and, in finite time with finite resources, and in a completely general way, compare their intelligences?
The answer might be "the paper doesn't get us any closer to that." So just say that. Otherwise you're being misleading, because you start out by presenting the problem in a simple way. Then you get complex when you answer it.