The latest "official" post about this which I could find was this Meta SE answer by Shog9 from 2015. I would personally summarize it as:
The system checks the impact of removing a to-be-deleted user's votes, examining both the total number of votes cast as well as the votes cast against individual users.
If either exceeds a potentially site-specific, non-disclosed threshold (all he said there is that it's "pretty low"), the deletion will be put on hold and requires a manual moderator review. During that, it will be ensured for example that the user was not involved in voting fraud. If that is not the case, votes will be preserved.
This post doesn't say anything about a threshold regarding the to-be-deleted user's own reputation. My guess is that this was the old filter (before 2015) which turned out to be not that effective.
The FAQ post about this was also very recently (couple hours ago) edited to mention the number of cast votes as criteria for whether votes will be preserved or not, instead of the reputation count. So probably all the help pages are wrong for a couple years now and nobody noticed yet until today :)