If somebody makes an Ad HominemLogical Fallacies argument like "Charles Darwin was racist, therefore evolutionary biology is false" and I counter with "By the same logic, the Nazis discovered that smoking causes cancer, therefore 'smoking = cancer' is false", have I lost the argument thanks to Godwin's Law?
Only amongst people who misquote Godwin's Law. There is nothing inherently wrong with referencing Nazis in an argument, and Godwin's original statement never said that there was. Any place that imposes a rule of "if you mention Nazis, you lose the argument" is making a lazy generalisation, on the basis that Nazi comparisons are usuallyAd Hominem attacks on your opponent.
Here's a simple rule: If Hitler/Nazis are relevant to the discussion it's not Godwin's Law. If Hitler/Nazis are used to demonize or insult the opponent it is. In that case it's making a comparison using the same logic as the other person.
Godwin's Law is an all-purpose shutdown. When someone uses it in a conversation unrelated to the Nazis or WW 2, it's a sign that the person is out of arguments and basically has lost the discussion.
In your example you didn't accuse the other person of being a Nazi, so Godwin's Law does not apply.
Actually, the other person has already invoked Godwin's Law by invoking Racism in a topic unrelated to race relations, at least according to the Pædo Hunt corollary.
Could one consider the outburst we've had lately with Jews/Zionists/Semites accusing people of being anti-Semitists/anti-Zionists whenever said people strongly disagree with them cases of Godwin's Law? And if so, why do people seem to accept this Ad Hominem as a viable argument when a Jew/Zionist/Semite is the one using it? note Unto my knowledge, this practice has become so prominent within Israeli politics that I can recall at least one case where another Jew/Zionist/Semite was faced with the accusation. Unto my knowledge the accusation struck home, too, and the accused man was effectively cold shoulder-ed. I, for one, was accused of anti-Semitism/being an anti-Semite when I expressed my personal opinion that I considered what Israel did to the Palistinians/People of Gaza during the 22 day "war" back in 2008 very objectionable and repulsive...
That seems kinda similar to N-Word Privileges. The Jews were the one who were slaughtered by the Nazis the most, so it's a shame some would use that to excuse their accusations but balk when another says the same thing to them.