Technically speaking, gtkfilechooserdialog.h isn't found anywhere in the minigtk source, and may not even be used in the official build at all (I don't know, I have no access to it)
Not many people compile XChat on their own for Windows, and even fewer use minigtk, so it is a bit of a leap to say that zed is specifically breaking things (if he is or isn't, I don't know, although I've yelled at him for commits before), but it is rather obvious that Zed doesn't want to give pointers out for something that could possible result in a "competing" version. Still, that is a far cry from intentionally breaking.
As to the specific question at hand, the use of gtkfilechooserdialog was added in
Revision 559, which was a) added prior to the creation of the guides in question (although the version check was removed after them) and b) used to make use of a new function, which wouldn't have then been ported to minigtk, as it already had something to use.
And shoot, I can compile XChat with that include commented out on Liniux, haven't attempted to run it, but it does compile.