GitLab Support Ben kindly helped me out here:
That paragraph in the documentation is specifically about issues: code commits are matched using the email address on the commit. I folks’ default email address doesn’t handle it, I would anticipate that the ‘add email address’ feature in the emails section under each person’s account can be used to map those across as needed, probably retrospectively.
associated their Bitbucket account
means going into ‘account’ in their user settings, and using the Bitbucket Connect feature under social-sign in. It requires their id.atlassian.com credentials, so can’t be done by an administrator. It might be the case that there’s an underlying identifier that’s brought across via the social sign in process.
The docs note what’ll happen if this isn’t in place:
a reference on the issue about the original Bitbucket author is kept.
Issues in GitLab are artifacts of the database, and aspects of each issue that refer to users map back to the user table. The importer therefore has to have some way to map from Atlassian to GitLab user table.
It can’t map to users that don’t exist in GitLab. And creating them would be undesirable, particularly as ‘active’ users in GitLab consume a license.
I would suggest creating a test project on Bitbucket cloud to see how this is going to work - get some colleagues to create issues, etc.