Pruning is a process by which a bitcoin node can limit the amount of disk space it consumes by discarding block files that have been validated and are buried by a certain amount of chain work.
Reindexing is the process of rebuilding chainstate using block data that already resides on disk.
From the original issue:
It appears that -prune is ignored during import (-reindex, or -loadblock).
For example starting with an empty datadir, prune=80000, and all block files passed in via -loadblock [doesn't result in pruning occurring.]