Apparently, the brain also burns lactate.

Lactate fuels the human brain during exercise Bjørn Quistorff, Niels H. Secher, and Johannes J. Van Lieshout Abstract

OP's paper gives at least some (but not many) examples where manipulations in glucose levels modified cognitive performance. However they mostly just observed that attention leads to lower glucose levels, which would also be observed if glucose would be one among many energy sources.

Also, before eating all that candy, note (from OP's paper)

One study administered glucose drinks to participants and found that poor glucose tolerance (indicated by glucose levels remaining high after the person consumed the drink) was associated with poorer performance on a dichotic listening task, which is another classic attention- control task and requires participants to ignore infor- mation presented in one earphone in order to track and process the information coming in the other ear (Allen, Gross, Aloia, & Billingsley, 1996).

I.e. high glucose levels lead to lower cognitive performance.