I don't think an Amiga is defined in one dimension, be it the OS, custom hardware or any other single aspect. It is the combination of these and the overall experience that defines it, IMHO.
No disrespect intended to the NatAmi team, or Dennis but if the native chipset were the sole deciding factor in the definition, then the NatAmi and MiniMig are no more "real" than UAE is. The only tangible difference being that they are a hardware, rather than software realisations of the original.
If, on the other hand, you base the definition on the overall user experience, there is no doubt that the minimig (and presumably the natami) are every bit as real as anything as commodore released. However, since that is a subjective definition, you have to accept that the other amiga compatible systems out there are equally "real".