Yes, you were right in saying they are not composting earthworms. The
aim of the experiment was not to raise them for composting, but for
improving soil quality by producing wormcasts. The second reason was
to grow them for observation instead of digging in the soil and
disturbing the organisms being observed.
I have witnessed a few cases where planting containers that have been
placed in the garden during the winter have successfully grown
earthworms which have entered through the drainage holes at the
bottom. I knew they came from the garden and were not initially in the
containers, because one contained used peat soil which I purchased
during the summer, and the other one had gravel. The case of the
second container with the gravel sounds strange: I find-out that
rotting material at the bottom, as well as a high rainfall that
preserved humidity made that possible. The area with the rotting
matter was teaming with hatchlings.
The question seems to be how much impact digging has on the
earthworms? If I pick-up the soil in which they live and place it in
the container, what are their chances to recover, and in how much time
?