Yes it is a phone in that it allows 2 or more parties to conduct a voice conversation utilizing technology over some kind of network. Fortunately, the article doesn't state that it is not a phone. It states that it doesn't require a phone in the sense that it doesn't need to connect to your phone in your pocket (most likely via bluetooth) to make/receive a call.

It has a built-in SIM so it stands on its own without needing to be paired to a phone. However, they refer to it as a mirrored SIM which means it can have the same phone number as your phone.

I fail to see the difference between a mirrored SIM and a cloned SIM (illegal in the U.S. as far as I know).