In what context do they refer to overweight people? Far as I know, but only been a member for the last 40+ years, they've never stated that ONLY the overweight ever get Type 2 diabetes.
Many outwardly slender folk are thin outside but fat inside, ie fat has settled around their internal organs rather than manifesting itself under the skin and therefore having visible fat. Ages since, Michael Mosley discovered he was TOFI, even though he's a doctor and reasonably healthy and fit - so for all you or I know, TOFI might run in your family cos it isn't one of the things any of us can easily check, but it's a certain fact that not everyone overweight or even obese, always gets diabetes - and it has been quite long recognised by now that there's a genetic element for diabetes.