It seems like a terrible assumption for this case.

Godwin's Law is largely a specific case of conversational drift (unless you start out talking about Nazis). Everyone starts out on topic, then gets more hyperbolic and distracted until you end up at some other idea, in this case Nazis.

So not only are comments in threads not independent samples, we're specifically looking at a proposition about non-independence.

He's formulating a null hypothesis - a (perhaps rejectable) alternative to the unspoken assumption in most of this thread, which is that Reddit, or perhaps comment threads in general, are disproportionately likely to include a mention of Nazis as they grow.