I was actually (closely) paraphrasing what Greg KH had said. Whenever in the past I had pointed out how extremely portable Linux is, someone always told me "Well, NetBSD is moreso." This has long given me a great deal of respect for the NetBSD folks (which I still have). It was interesting to hear someone deep in the know say that Linux has actually managed to surpass NetBSD, and when.
As for what exactly he was referring to, I suspect he means the kernel and associated modules. To be fair, Debian (as a complete OS with kernel and userland) supports somewhere around a dozen architectures officially, plus a pile more in special vendor variants, experimental trees, "official in next stable release", and so on.
In fact, I've heard that a large part of the portability of *nix userland is due to the combined efforts of NetBSD and Debian, who between them are the most rigorous proponents of portability in the *nix world. Debian, I know, is ruthless in getting packages to compile in environments the authors never tried or even intended. I suspect NetBSD is similar, though I don't know first hand.