No it's not, it's a rewrite like the others.
Yes it is. I know because I have access to the source code. Hyperion ported AmigaOS to PowerPC. While doing so, they replaced some components (like Exec), and enhanced others.
This is beside the point, though. Even if it was a rewrite, it would still be AmigaOS, just like MacOS X is MacOS because Apple say so.