I think you want Step 2 to determine WHICH books get this treatment.
That is done BEFORE entering
Bulk Mode MDE. A
Search (criteria) is the typical way
Step 3 is Select All (results of #2)
Step 4 is the 'copying' of the old
Authors ->
#various_authors as BR has shown
Step 5(a,b) is to replace the old
Authors with a severely truncated value.

I use
Editors_name (ed) or
<Various> in the Authors field when the original list of Authors is >2 (I started using Calibre with A filename based reader device and needed a reasonable length file name)