I once got an XML file from an integration partner where the whole thing was XML escaped (all the tags looked like <node>value</node>) because they had embedded

I once got an XML file from an integration partner where the whole thing was XML escaped (all the tags looked like <node>value</node>) because they had embedded it within an outer "envelope" XML file. They saw nothing wrong with this and argued when I questioned it. I wonder how they were planning to express escape sequences within the inner XML document that was already escaped...