It probably is an error. Something must have gotten scrambled up between Seagate's PR department and the team that manages technical specs/documentation.
Maybe someone mashed up a different single-platter drive being released (one of the "Backup Plus slim" models?) & the fairly thick (20mm) 4 TB "Backup Plus portable" drive. Or maybe the single platter drive is one of the "Backup Plus Desktop" models (3.5", with AC adapter... Maybe the 2TB model is a single platter).
We'll find out once they actually go on sale soon.