Super naive tweet, there is not even a picture of the drive inside, not an output from smartctl or something.

Seagate makes multiple lines of of HDD drives - which one is inside? Is it the same line of drives that's in Seagate-branded external drive or not?

There are very few manufacturers of hard drives, and LaCie isn't one of them. They package drives in very nice cases and cater to the Mac crowd. This is far from a mystery, and certainly not worth noting.