What is inside dsofile that could not be written in plain VBA?
As I didn't write dsofile, I have no idea what's in there, other than a bunch of C++ to work with compound objects! There may or may not be equivalent API functions that you can use from VBA, but just because something is possible for Microsoft, doesn't mean it's available to you as a VBA developer. In that article, you will note the section that reads:
"To help developers that are interested in reading document properties, we have provided the following two interfaces to manage property sets: <TABLE class="list ul"><TBODY><TR><TD class=bullet>•</TD><TD class=text>IPropertySetStorage
</TD></TR><TR><TD class=bullet>•</TD><TD class=text>IPropertyStorage
</TD></TR></TBODY></TABLE>However, some high-level programming languages may have trouble using these interfaces because the interfaces are not Automation-compatible. To resolve this problem, developers can use an ActiveX DLL, such the "DsoFile sample" to read and to write the most common properties that are used in OLE compound documents. This applies particularly those that are used by Microsoft Office applications."