I wonder if there were union work rules at play concerning just who was allowed/not allowed to perform the task of bolt insertion?
No — the issue is that Boeing managers decided to describe the operation as “Opening” the panel instead of “Removing” the panel. See if the panel is removed, they are required to verify and document that the bolts have been reassigned, which takes time and money. In contrast, if it is simply “opened”, there are no bolts to worry about and all that pesky paperwork and useless inspections can be avoided.
This one is squarely on Boeing managers cutting costs by cutting corners.