We use the following code to export the attrubute:
<#if clazz.abstract?exists && clazz.abstract>
abstract="true"
</#if>
but "abstract" attribute held into PersistentClass.isAbstract. I guess developers of freemarker check only get+Fieldname and is+Fieldname, like java convension says.
So, the working code is:
<#if clazz.isAbstract()?exists && clazz.isAbstract()>
abstract="true"
</#if>

Patch and unit test will be attached.