That might have been an error in Arnett, or that might have been my error, reading it as body length when Arnett said, or at least meant, wingspan.
Dr. Oswald did identify this as S. amiculus.
Thanks for your comments. I will leave the original caption, but add a note to see comments.

Someone suggested one of my S. barberi observations on iNat should have been identified as S. amiculus instead. I've not found a good illustration of the essential difference in wing venation between the two. Rereading Oswald, I see he says, "Radial cross vein absent, very rarely joinging R1+2+3 to R4+5." I find illustrations of S. amiculus which shows a cross vein connection R2+3 to R4. That cross vein is absent in my observation. Is that the cross vein being referred to by Oswald? More distally, there is a cross vein connecting R3 to R4 both in the illustration and in my observation where I have added an illustration.

Interesting comments, but I do not have anything useful to add. The ID here was based on Dr. Oswald's expertise, and not mine.