their blood being like mayple syrup in terms of viscosity,

Lol. Where did you hear this? I don't mean to offend, but it made me laugh. Their blood sugar levels, although physiologically high, are nowhere near the levels needed to cause a dramatic viscous change like that. What actually happens is that the high sugar levels in the blood lead to thickening of the walls of blood vessels due to a number of sort of complicated things. We're not entirely sure why it happens, but we think that in part, sugar in the blood binds with proteins and these sugar-protein molecules accumulate in blood vessel walls. Thicker walls means thinner lumen, which leads to less blood flow, especially in very small blood vessels like capillaries. Nerves are largely fed by capillaries. Without blood flow, the nerves don't work right, so diabetics can't tell that their feet are injured if they get cut or burned. They don't know they're injured, so they don't properly care for their wounds. Repeated trauma with slowed healing (also due to low blood flow) can lead to necrosis, gangrene, and infections, and eventually the leg needs to be amputated. This is why we advise any diabetic to perform a daily foot inspection for any injuries that need to be properly taken care of.

Additionally, it has little to do with the amount of sugar in their diet, but more to do with a combination of insulin deficiency and insulin resistance. That's more complicated though.

Just for some perspective on this (warning: very amateur, unscientific analysis)

192 grams of sugar are found in 1 cup, or 236 mL of maple syrup. A fasting glucose of 126 mg/dL is diagnostic for diabetes mellitus. Let's make that 300 mg/dL for a very poorly managed diabetic who just ate a big meal.

For maple syrup, 192 g of sugar in 236 mL is equal to 192000 mg of sugar in 2.36 dL. That's a level of 81,355 mg/dL. That's a sugar level almost 300 times higher than a level you would ever expect to see in even the worst-managed diabetic patient.