I think the study is saying that if you want to have a slightly lower GI summer pudding you could make it with white bread and strawberries (rather than some of the other berries) Alternatively you could make it with either white or rye bread with a delightful mixture of strawberries, bilberries, cranberries, and blackcurrants
The researchers say that there are test tube studies to suggest that the polyphenols in berries may reduce the absorption and digestion of starch.(so as Catherine says alter GI) They were testing the effect in healthy women
Strawberries, bilberries, lingonberries, and chokeberries all significantly reduced the amount of insulin produced compared with that produced by white bread alone,( this presumably means that raspberries and cloudberries didn't, )
The berry mixture with either white or rye bread also resulted in a significantly lower production of insulin.
Interestingly, it was only strawberries with the white bread or the berry mixture with either white or rye bread that actually also reduced the glucose response ( 36%; white bread/strawberries, 38% white bread/berry mix ; 19% rye/berry mix
( remember though these were women with normal glucose mechanisms and that though the rye bread may seem to have a worse result ,it already had a lower glucose response. White bread is high GI, rye normally lower)
Seriously, I think what the others said about eating low GI fruit or berries for dessert is probably the best advice.( I'm T1 but that's what I do and thinking about it, I don't tend to add any extra insulin for berries or a small fruit salad eaten for dessert. )