Huh, that’s a good question. I just assumed scifi magic, they make the rules, so they do what they want, but it’d have made more sense if she were Hullen too. What a twist that’d have been, if they realized, before they met Aneela, that Dutch is Hullen too. But obviously that’d have never worked for the plot, plus, they’d want to separate her and Aneela and Aneela’s already Hullen.
I guess they just say that since Dutch (and in fact Khlyen in season 5) are memory made form, they’re whatever they were in the original, exactly, like atom by atom copies of what they were. It wouldn’t make sense that just their blood and brain would have the Hullen modifications, since that’s actually a separate infection of sorts.