You guess is 100% correct.

The MixedCodeDocument class was designed to be able to parse a text that contains two languages, that is, classic ASP, ASP.NET, etc. hence the name :-)

Originally the Html Agility Pack was used in a tool that is capable of processing and transforming a whole tree of various files, including HTML and other types of file. If you needed to replace only the HTML parts for other files, this class helped you split code & markup and. Separated code and markup blocks can then be parsed by other means.

I don't think anyone's using it today :)