As much as I'd be prone by education and origin to go with this (as someone born & raised in ex-Yugoslavia), that is just not true. There was a bloody civil (ðnical) war going on and none of the sides involved were willing (or able) to cause significant trouble to Germans.
* Germans were kicked out of Belgrade by Russians* in 1944.
* Some 20,000 people died on Srem Front in late 1944. on partisan side unsuccessfully trying to breach Axis defense. Russians were the ones who smoked that too.
* Partisans were more interested in hunting down retreating Ustasha and Chetnik forces (and did some 'good killing' in Austria when they caught them) than fighting retreating but still formidable Germans.
Yugoslavia was left out as part of Churchill-Stalin agreement (50-50) and not because of any local capability to resist Russian occupation.
* When I say Russians I mean mostly Ukrainian (for whatever value of Ukrainian) sourced forces.