Jail break doesn't add features. It removes restrictions on the iOS for the features apple buried or removed access too.
Jail break devs in this case didn't add a small collection of BT mouse drivers and inject the UI code to read mouse movement.
I understand Jailbreaking in itself doesn't add any features. It simply blows a hole in the walled garden. As far as mouse support by simply jailbreaking and pairing a mouse? If that's truley the case, i'm surprised. I would assume some software would have to be installed post jailbreak to allow this functionality. Considering there is no UI for it, for example.