I too would generally regard observations of black ravens as being weak evidence that all ravens are black.

Weak evidence, but evidence nonetheless. I read the essay again, and it appears that what the author means is that there exists a case where observing a black raven is not evidence that all ravens are black; the case he specified is one where the raven is picked from a population already known to be consisting of black ravens only. In some sense, he is correct. Then again, this is not a new observation.

He does present a case where observing a red haring constitutes weak probabilistic evidence that all ravens are black.

So, my disagreement comes from my misinterpretation of the word "may".