Whatever happened, it was not "collections" that saved you from disaster.
You didn't tell us what happened with the external HD. Was it fixed and the files restored to their locations on the external HD? Could you please explain "Upon reopening LR...", exactly what did you do between the time the hard disk failed and the reopening? Was the external disk attached? Not attached? Did you do anything else in this time?
Regardless, exporting is not the solution. You need to be making regular and automated backups to a different physical disk of your Lightroom catalog file and also all of your original photos.
The external hard drive failed while working in LR on the exact Collection in question.
I had been working on this Collection for many days and had exported many JPGs from the DNG working files also stored on the failed drive.
All the exported jpg files were written to the failed hard drive prior to its demise.
After the drive was disconnected I reopened LR with the failed drive no longer connected.
And to my surprise there was the Collection with the JPG files as originally exported.
All the DNG files are missing however. (no issue, I keep the MSD cards as backup)
I'm going to guess, because really I wasn't sitting at your computer when it happened. I think that your exports were accidentally placed on the internal drive, and so when you re-opened Lightroom without the external drive connected, the files on the internal drive were there where Lightroom expected them to be, and thus Lightroom finds them and can use them.
This has nothing to do with the fact that the photos are in a collection, which is irrelevant here.
Again, you need to make backups of your original photos AND the Lightroom catalog file, to a completely different physical disk than the originals.