The link I posted above shows that they do run hotter - someone actually tested it. Significantly hotter? No. But that was with a single drive and OP should
make sure that there are gaps between each unit, otherwise having ventilation holes on the side absolutely will mean heat can't escape.
On a different note, I have one of the pictured Seagate drives at home, and as you can see in the picture, you're wrong. I assume you haven't been to work recently and have forgotten, but they absolutely do have ventilation holes at the sides.
The Seagate Backup Plus 8TB is the latest addition to their portfolio of external desktop drives, offering users with the largest available single-drive solution available on the market. The availability of an 8TB model is certainly a very important release for many, as data size and storage...
www.storagereview.com
You still haven't really answered the question as to why you think that there's a correlation between a seagate external enclosure being designed to be laid flat (and only designed to be laid flat...) and a WD enclosure that was designed to be stood vertically being able to lay flat.