It it possible to opt-out of the check for PrivateAssets using the CakeContribGuidelinesCakeReferenceOmitPrivateCheck setting:
(Keep in mind, though, that it is not recommended to opt-out of this feature)
<PropertyGroup>
<CakeContribGuidelinesCakeReferenceOmitPrivateCheck>1</CakeContribGuidelinesCakeReferenceOmitPrivateCheck>
</PropertyGroup>