Osborn and Mook (1921) actually synonymize Amphicoelias latus with Camarasaurus supremus, but Carpenter (1998) considers it a synonym of Camarasaurus grandis because it was found lower in the Morrison Formation and the deeply concave articular faces on the caudal vertebrae are more consistent with C. grandis. Tschopp et al. (2015, 2017) recover Amphicoelias as either a basal diplodocid or an apatosaurine more derived than Brontosaurus excelsus. Therefore, you may be right to point out that Amphicoelias shares a circular femoral cross-section with Diplodocus, but the paper should take into account the differences between Amphicoelias and other diplodocids jotted out by Tschopp et al. (2015).
Carpenter, K., 1998, Vertebrate biostratigraphy of the Morrison Formation near Canon City, Colorado: In: The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Edited by Carpenter K., Chure D. J., and Kirkland J. I., Modern Geology, v. 23, part 2, p. 407-426.
Tschopp, E.; Mateus, O. V.; Benson, R. B. J. (2015). "A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda)". PeerJ. 3: e857. doi:10.7717/peerj.857. PMC 4393826 . PMID 25870766.
Emanuel Tschopp; Octávio Mateus (2017). "[Osteology of Galeamopus pabsti sp. nov. (Sauropoda: Diplodocidae), with implications for neurocentral closure timing, and the cervico-dorsal transition in diplodocids].". PeerJ. 5: e3179. doi:10.7717/peerj.3179.