Hi Matthais
I may not have the latest OS but I can usually get the antiques going.
Fired up an old copy CS4 and was surprised that you can Export as INX or IDML, thing is CS4 was designed to backsave as INX (for CS3). Before CS5 came out what would be the point of IDML for CS4 ?.
CS4 needs to open IDML from future versions but only needs to open INDD from itself or earlier versions.
Because CS5 CS5.5 CS6 CC etc etc can all open INDD saved CS4 files, the IDML option in CS4 seems very redundant, behind the scenes if Adobe chose to write INDD into the CS4 IDML no one would ever know?
Sure Exporting as INX or IDML is an option to clean up bloated or corrupt INDD but its a weird one for CS4, for example I don't recall Quark having a feature to Export in a format for future versions, unless you count the Document Converter program.
Incidentally there are currently a few people finding it difficult to open CC2018 generated IDML in earlier versions of InDesign, I've not come across anything myself but Its worrying for me because I need CC2014 until I get reliable updates for my Plug-Ins.