The XFA (XML Forms Architecture) was introduced and used for web form enhancement and were saved internally in pdf files. PDF 2.0 has already depreciated it.
Although XFA can make use of PDF, XFA is not tied to a particular page description language. -wikipedia
Hence, PDF is not a XML based file format. Although some additional feature which are tied with pdf like XFA, do use XML.
u r right. Nice catch! thanks 👌