Dunno the finer points of terminology, but this essentially developer-unlocks your phone, i.e. it uses the same mechanism that's used to let you sideload your own apps for testing if you're officially registered as a developer. The difference being that it's cheaper (and possibly less of a hassle to get?) and doesn't actually let you put apps on the app store. I think that's different from the iPhone jailbreaks which don't play as nicely with the security mechanisms, but again I'm not really up on the details there.

Even so, they (ChevronWP7) appear to be calling it an "unlock". Just to increase the confusion, I guess.

In WP7 world, it's always have been called unlock. The career unlock is called as SIM unlock and this one as developer unlock.

It's a confusing verbal inconsistency, to be sure, but I think it is at least preferable to the term "jailbreak" in this context because it is not adversarial in nature to the platform maintainers.

Couldn't some homebrew software enable carrier unlocking?

I don't know about rooted Androids, but jailbroken iPhones can be unlocked.

Not all. Jailbreaking requires exploiting a (variable number of) holes in the system, while carrier unlocking requires exploiting the baseband. The former is a prerequisite of the latter, but the carrier unlock is not always possible even if you're jailbroken as it depends on holes found in the baseband, which depend on the baseband version.