The Foundation sees this as a contradiction to the EU’s own interoperability goals. Although XLSX is standardized as OOXML according to ISO/IEC 29500, Microsoft’s implementations often deviate from the specifications. Furthermore, features often change undocumented, which complicates compatibility with open-source software such as LibreOffice.
I kinda get it though. I think every single time in my life I’ve sent a document in the non-Microsoft format I’ve got a reply saying they couldn’t open it. That’s from LibreOffice and from Mac.
Either the person is lying, because MS Office claims compatibility with OpenDocument files, or it isn’t actually compatible and Microsoft itself is lying.