That's a very good point! I didn't realize that the timestamp was block time, not creation time. To confirm that, I checked against the posts I have saved and regenerated the graph I have shown above using 3 second bins. I would attach it here, but it is predictably identical to the one above! I still maintain that ~60 votes in three seconds (~20 votes in one) is moderately suspicious (although certainly less so).
That's a good suggestion! I think for my next post, I may look into clustering voters to see this kind of grouping. I'd also like to look into voters based on SBD value they've added to the post.
Thanks for reading!