Godwin's Law is that basically any internet argument will inevitably descend into someone comparing somebody they disagree with to Hitler or the Nazis.
That wasn't what happened on this thread.
Because that's not Godwin's Law. The comparison isn't of the person we disagree with. It's a comparison with Hitler or the Nazis, in general. And that certainly happened in this thread, when someone made a question about a comparison between the Prince and Hitler.
It is a clear example of reductio at hitlerium (spelling?) where you claim he isn't racist because he wasn't like Hitler.
BTW, I'd expect people to understand these days that there were many shades of racism and many forms. You can be racist and have non-white friends. You can treat some non-white well and others not. You can even employ without looking at race and be racist.
And there's a difference between being a racist and being racist. Some behaviour can be racist, even if the person isn't a racist. Many forms of racism have been learned growing up and even those of us who think are not, should sometimes pause and verify our own biases and prejudices.
In that sense, the whole argument about Prince Philip being racist or a racist is sterile. He had behaviour, or at least words, that is considered racist. How much he was at heart, we can't say.
But remember that he could be racist and still be nice to the Obamas. It's like saying, I'm not racist, I have a black friend. 
But a few ill placed comments do not make him a racist. Not as a defined characteristic.
In my opinion most of the pps all recognise his failings, just disagree on what to call it. He's not a devil or a saint. Shocker.