The schema defines default values in property definition types like this (string property type example):
<xs:element minOccurs="0" maxOccurs="unbounded" name="defaultValue" type="cmis:cmisChoiceStringType" />

This implies that a property definition type could have multiple default values which doesn't make sense. It should be:
<xs:element minOccurs="0" maxOccurs="1" name="defaultValue" type="cmis:cmisChoiceStringType" />

Apart from that, cmisChoiceStringType defines some attributes and a hierarchy that don't apply to a default value. Shouldn't it be a simple value/list?