I googled around looking at the opposite direction, are t-cells small enough to get flushed out by dialysis? Could not get a straight answer. There must be a lot of money to be made in dialysis; there are a lot of "popular science" type articles about it. My guess is we'd have to try a level of indirection, red blood cells aren't lost, so relative size of t-cells vs red blood cells would probably answer that.

The other idea I have is MDs are not overly dumb, so if its possible for natural or artificial kidneys to filter the bad stuff to the outside while keeping the good stuff inside, they would probably just absolutely flood the patient with IV fluid. The original article was very detailed about the experience; this didn't make the article, so I don't think they were doing it.