That's pretty much what they are. Hard drives are binned based on the yield of each production run. Server drives get the best lots, then tier one OEMs, etc. External drives are the lowest bin before the trash bin.
I'm actually not sure that's true. I run a NAS with 12 8TB drives, and not a single one has failed over several years. Nothing in SMART log either. All "shucked" cheap WD My Books. Running raidz-2. Sure, an anecdote, but how do you even "bin" something like this? It's not like you can vary voltage or frequency or fuse out busted cores (which is how they bin the CPUs).