Wisconsin doesn’t have party registration at all and hasn’t for a while (though it did when I first registered, and it cant be undone or changed now, leading to the appearance that we just have a huge population of independent voters (about 1/3 at this point), and no other changes in the last 20-ish years… so if it only includes people who are registered, it misses a lot of important data. Idk about other states, I think there’s at least one other that doesn’t do party registration, but a lot of people opt to be independent if given the opportunity because they lean left and dems kinda don’t.

So I think that interpretation is probably accurate.