Not a doctor but it's because when you are diabetic the angiotensin system gets out of whack. Angiotensin is what is part of what regulates sodium and potassium in your kidneys which is what controls blood pressure.

https://en.m.wikipedia.org/wiki/Renin%E2%80%93angiotensin_sy...

I think the salt consumption is a bit of a red herring and it's better to fix the metabolic syndrome.