> Been a Linux user for almost a decade. [...] No, it's because I was dumb enough to think I could just DD a Windows ISO, like every other ISO I've ever encountered. Microsoft are special, I need to use special tools.

You haven't been a Linux user for long enough. The Linux ISO you can "just DD to a USB key" is a hybrid ISO, which is actually a hack which has both a HDD-style partition table and CD-style headers, carefully set up so that their data areas overlap. Early Linux ISO images, like every other ISO image you would encounter back then, weren't hybrid; they could only be booted from a CD drive, and you had to use special tools to convert them to something which could boot from a USB key, or you could instead DD a floppy disk image (helpfully found in a directory within the CD image) to a floppy disk and boot from it (it would still read all the installation files from the CD).