If I am understanding the question correctly, admins ability to have an easy tool to use to recover space has always been lacking. If I am not mistaken it's been that way ever since I can remember. So there is no 'official' way. Think it has to do with private files which is hidden from admin level users - that's where you found that zip, correct?
What I do ... once I know the user id, login as, then do a 'fake restore' ... start the restore process whose first step is to find a backup file ... one then sees the 'private files' area and thus can delete - and save. Then cancel the restore process.
Large site with space issue ... that is very time consuming.
This is where moosh might be able to help. See: https://moosh-online.com/commands/
That's why the admin makes the 'big bucks', right!
'spirit of sharing', Ken