I think that Ni-Ti and Ni-Fi (and vice versa Ti-Ni, Fi-Ni) could be equally abstract, but it is an interesting question. Ni-Fi would be explore abstraction in a more personalized, unique manner that would be more difficult to communicate in a way that accurately represents it to others. Ni-Ti would likely be abstract in a slightly more definable way because it would reference more universal logical principles. Ne also plays a role in abstraction, so one could make the argument that Fi-Ne and Ti-Ne (and vice versa) could also produce abstractions.
I do consider myself pretty abstract on the inside, but it does tend to be an extension of the outside world. I see what is concrete and abstractions form from it. I have trouble creating completely independent imaginary landscapes. I wouldn't be able to write complex fantasy stories like both my brother and sister like to do, but I do have these flashes of abstractions and spent countless hours trying to comprehend the nature of the universe, other dimensions, etc. Also when I see strangers and am in a certain state of mind, I will see their expression, the patterns of their movements and suddenly the entire ranges of emotional expressions flash through my mind where I can see them expressing anger, joy, sadness, etc. I get strong impressions whether the person explodes with anger or internalizes it, etc. I have no idea if these impressions are accurate because it happens in the moment on viewing random people, but it is an abstraction of what I'm see concretely, it's like an extrapolation of suggested core patterns. I think it might be a Ni-Fi loop inside my head, but I'm not entirely certain which functions it involves. I do know that my attention wanes when viewing completely imaginary systems and it is easier for me to extend the concrete fragmentary impressions into wholistic, complete abstract patterns. Whatever that is, it is how my brain works.