While the title matches the article, it really isn't descriptive of the content of the article (which is all about modifying and flashing the ROM, there's no 'jailbreak' yet).
He achieved full code execution on the device. You can argue that it's not a jailbreak because there wasn't much of a "jail" (checksums, rather than crypto), but I'm not sure what more you would want in terms of capabilities.
I disagree. The github repo he links to in this writeup contains everything necessary to install a custom firmware to the Fitness Band. He even includes the binary itself, which is probably not too smart(it would have been better to write a script to download the binary direct from MS's server). What more do you want? Cydia?