It looks like it did let me do it without a key. I don't understand. How can Microsoft determine if the software has been licensed for this hardware?
This is not the same question. It is the same machine, but an entirely different use case and operating system.
What's the exact name of the ISO of Windows 10? From where did you download the ISO? What was the exact error message that you encountered while trying to perform a clean install?
Well, you just answered your own query, right? You can press the “I don't have a product key” button and continue the installation process with your choice of the Windows SKU. Should I mark this thread as solved?
I don't see how to market as solved. I'm still looking to understand why I wouldn't need a product key? Are early versions of Windows now free?
Due to the fact that you've experience of installing Windows since Vista-days, I presumed you're aware of placeholder keys. Microsoft always defines some key-ranges that are meant for allowing users to skip the serial prompt. When you select the “I don't have a product key” option, the Windows installation wizard applies the SKU-specific placeholder key on the fly. It won't activate the copy of Winodws, but enough for evaluating the OS.