I found another paper [1] that implies that "impaired glycemic response" is measured by an "oral glucose tolerance test" and that high levels of blood glucose in the two hours after drinking a glucose solution are what they mean by an impaired response. The graphical abstract [2] from the paper discussed in the Science article has "glycemic response" graphs, which I assume are from this oral glucose tolerance test, although I wasn't able to access the paper's PDF.

[1] https://www.researchgate.net/figure/Impaired-glycemic-respon...

[2] https://www.cell.com/cell/fulltext/S0092-8674(22)00919-9?_re...