It’s not about a media player ‘attempting to execute random code’ - an exploit is found which lets it run a command that it shouldn’t. You used to be able to jailbreak phones by loading a .pdf file that used an exploit to gain root privileges and execute code. It wasn’t a feature of the PDF reader. It was a bug that could be exploited when a specific string of characters was entered to effectively crash the pdf reader and let it run its own code instead.
A txt could easily contain malware - any file could.