Hello Bernard,
Follow-up thought: since gdb is a debugger, does this mean I should have compiled the original executable with debugging symbols? (Something I almost never do for a production executable.)
Almost certainly, yes. Though maybe not all off them but enough that say "nm exe/cactus_sim_ET_2021_05_BC" reports something for cactus_source (eg in my case nm exe/cactus_sim | grep 'cactus_source$' -> 0000000006fdc1c0 D cactus_source). Gdb needs to be able to resolve the "cactus_source" symbol into the actual address. Otherwise it will be hard to find out that correct starting point for the linked list of tar file fragments.
If you can still run the the executable then running it with a minimal thornlist that says
ActiveThorns = "Formaline"
will write out the source code tarball.
Yours, Roland