For what it’s worth, this sort of connection doesnt seem to be strictly limited to the iPad Pros (otherwise, why have a Lightning-to-USB socket adapter?). I got Apple’s Lightning to USB3 Camera Adapter, so my nieces could load movies from a flash drive onto their (32 GB) iPads. i had plenty of success testing this adapter with my iPhone 8, using the external power connection (it wouldn’t run a 128 GB USB3 SanDisk Cruzer flash drive without the power connected). I expect the limitations for external hard drives are simply power requirements - the external power connection on the adapter supplies the iOS device, thus giving it enough backup to run the flash drive, but not enough for a hard drive (the iPad Pro’s USB-C gets rid of these problems).
But the conversation between iOS and a flash drive is essentially the same as between iOS and a hard drive - it’s all a matter of “gimme block number X”, “okay, here is block number X”). If you plug the hard drive into a *powered* USB hub, and then connect that hub to the “camera” adapter, it should be fine (I’ve seem the same sort of power limitations on Raspberry Pi’s with the same fix - plug the drive into a powered hub, and plug the hub into the Pi).
The other tidbit I ran into was numerous suggestions that iOS had trouble with files on an EXFAT formatted drive over a certain size (several gigabytes). The solution to this is to format the external drive as APFS. I did this (since movies are frequently 4+ GB), and it seemed to work fine. It’s possible the iOS bug relating to large files on EXFAT drives is fixed now, but I didn’t want to take any chances.