I do wonder if the 4TB I have came from an external enclosure like that, as I bought it "tested but not used" on ebay. I have to admit, I used to wonder why the typical consumer portable backup drives were cheaper than bare drives I could buy (per capacity) and I guess I now realize there is an actual reason why they are sold as such (i.e. backup not general purpose use).
I never came across the kind of price difference with 4TB drives that I did with 8TB drives, which was what caused me to research them and learn about SMR. As you observe, the cost of external drives can be cheaper than bare internal drives and for the most part whenever I've investigated (Newegg reviews can be a helpful starting point) the actual drive is the same, just the warranty may be different.
I don't think I ever saw Synology approve an SMR drive for their NAS units, but as far as the practice of pulling external drives for NAS use goes, I'm fairly sure I recall that Backblaze do this regularly. As many are already aware, Backlaze publish very interesting data on the longevity of different drive models - one of the reasons I've never been tempted to pay the premium for things like WD Red drives, versus using the cost difference to keep a spare drive on hand for RAID repair / recovery.
If someone suspects that a drive or drives may be under-performing and no health check warnings are being reported, using an external USB to SATA adapter to check each drive with a tool like Crystal Disk Mark can be enlightening. Before I sold the drives from my last NAS, I found that three of the eight were registering faults that could have been the beginnings of trouble. Those three were used for target practice!