why called them hedgehog channels?

It would be wrong to call them lightning channels because they violate the lightning specifications. I like to use different names for different things so that people can have clarity about which is which.

Wouldn’t it be better to frame it as an extension to lightning?

I think you can do both: hedgehog channels are like an extension that you can attach to a lightning node (e.g. via a bridge server) to gain additional features