You're probably right, I forget which way around it was, but if I was to take an educated guess, I'd say that SMR on the lower capacity models lets them cut costs because the added density of SMR lets them use fewer platters?
I'm not sure why SMR would save costs on lower capacity drives but not higher capacity drives. It may be a technical reason, but I'd also find a corporate-strategic reason to be plausible -- i.e., they were like, any consumers that give a shit about something as technical as SMR vs CMR are probably the ones buying our high-capacity drives.