The Canadian and Mexican studies are invalid because they use spot urine to measure fetal fluoride exposure. Spot urine of the pregnant woman can't measure her chronic fluoride exposure. It can't tell us anything about what the fetus was exposed to. These are studies, regretfully, that might as well not have been written. They are not useful because they are invalid.
I think you're referring to the Canadian and Mexican studies that were excluded from the meta-analysis.
If you read the paper, two Canadian studies and three Mexican studies were excluded, but not all of them were. Table 3 only shows results from studies that were judged to be robust.
That's not accurate. Table 3 shows negative impact from data collected in Canada and Mexico as well.