I've heard somewhere that docx is actually gzipped xml. But I never really confirmed that myself. But it's binary once gzipped, so your point still stands.
docx and epub are zip files, you can rename them with a .zip at the end and open to see what is inside. It might not be as simple to zip them back, at least for epub is very important to zip the files in a certain order but I forgot the details, but is easy to do from command line.
The xml will probably need to be run through a formatter to be readable. You can type `:%!xmllint --format -` if you have xmllint installed.
Now prepare to spend several hours trying to make sense of the xml. :-p
Definitely binary once compressed, though, and even when extracted not an easy format to parse. It might be XML, but it’s still representing the full complexity of an MS Office document.