This task is about providing a map between legacy Wikimedia Bugzilla usernames, and Wikimedia Phabricator usernames.
Approaches, from the best:
Note about the point n.2: this may be useful:
https://gitlab.wikimedia.org/valeriobozzolan/yet-another-bugzilla-parser/
Accounts were linked based on email addresses, there wasn't datasets per say.
If people added additional email addresses to their phab profile to match their previous email addresses, there was a cron script to update the actions that were assigned to bzimport.
Moreover, this is invalid, since the closerPHID is already known (sometime...). So, no need to look at Bugzilla.
I was super-somehow confused by the examples mentioned in the original description of T107254 (T2002 and T2007) that do not have the closer. But having no closer on a closed task is just an unfortunate desired situation: the related closer was probably not imported from Bugzilla (by the tasks mentioned by Aklapper) or, just none was really a closer.
Thanks for this epifania