I think a better way of presenting it is in a different order:
I think insect suffering might cause a million times more suffering than human suffering
But that’s not the important rhetorical point that the claimant likely intends to make in this case. EAs know that insect suffering sounds obviously crazy to the uninitiated, so the likely target audience here are people who already think that animal suffering charities of some sort (e.g. factory farming-related) satisfy the “effective” criterion, which is equivalent to thinking that animal suffering (at some scale) could be more important than human suffering. From there the claim that suffering among one subset of animals swamps all other by orders of magnitude isn’t crazy at all.
But that’s not the important rhetorical point that the claimant likely intends to make in this case. EAs know that insect suffering sounds obviously crazy to the uninitiated, so the likely target audience here are people who already think that animal suffering charities of some sort (e.g. factory farming-related) satisfy the “effective” criterion, which is equivalent to thinking that animal suffering (at some scale) could be more important than human suffering. From there the claim that suffering among one subset of animals swamps all other by orders of magnitude isn’t crazy at all.