Hmm there is a difference between a unknown and a crank. Zhang was a trained Ph.D. mathematician with an academic lecturing position. Now I didn't even finish my Ph.D. in physics but I know enough about how to do research to not submit a proof that is not rigorous enough to warrant peer review. One should not refuse to peer review a unknown researcher with the right credentials/trainings who make serious arguments in line with academic practices.
Same as you this is not a comment on Dr. Granville. I wrote in response to your comment on how the field is typically cloistered among a few insiders. I don't think that justifies refusing to review an unknown (as in not famous) researcher who demonstrates training and seriousness (which fortunately they didn't in Zhang's case but Zhang also was extra careful in making his ideas crystal clear). This happens too rarely for the insiders to claim undue burden. Cranks are very obvious to identify (a lack of literature review and understanding of previous works is a tell-tale sign).
Cranks and unknowns often look alike. Heck, researcher who diverge just a bit from accepted doctrines often look like cranks until either they repent or their approach bears obvious fruits.
Hmmm, that is plausible, but I can't think of any modern example where some deviation has resulted in a qualified researcher being categorized as a crank-- ignored maybe, or perhaps dismissed but definitely not called a "crank". Actual cranks are very very easy to spot and people won't bother wasting effort addressing them.
I'm not sure. Crank is often used as an ad hominem in science, so it is difficult to tell mudslinging from real crankiness, it depends on the sincerity of the one applying the label, and also on the severity of the crankiness. And many people have trouble with groking more benign eccentricity.
Same as you this is not a comment on Dr. Granville. I wrote in response to your comment on how the field is typically cloistered among a few insiders. I don't think that justifies refusing to review an unknown (as in not famous) researcher who demonstrates training and seriousness (which fortunately they didn't in Zhang's case but Zhang also was extra careful in making his ideas crystal clear). This happens too rarely for the insiders to claim undue burden. Cranks are very obvious to identify (a lack of literature review and understanding of previous works is a tell-tale sign).