You are right about the basic idea.
However, while that basic idea sounds simple, it turns out to make life very difficult for the backup and restores code. To get it to work reliably in all cases had taken a lot of extremely clever work from Mark Johnson over the last year or so. (All the cases meaning things like - backup restore to the same Moodle site or a different one; backup being a whole course or just one activity; ...). Fixing it properly in some cases required building on the new question bank stuff in Moodle 5.0+, so things could not be fixed in earlier versions.
Regrettably, it is not all perfect yet. For many years, there were bit, know, issues in question backup and restore. I think we (Mark) has now fixed those big issues. But, then what happened is that rather than everyone knowing that Question backup and restore was a mess, instead people started to expect it would work, but then notice subtle edge cases that were still wrong. Thanks to everyone who found the time to report it when they noticed one of these problems. A lot of these things have been fixed, but there are still about half a dozen left. They should all be linked from this Epic MDL-88134 - along with other things.
If anyone is in a position to fund development work to get these remaining issues fixed, please reach out to Luca Bösch. And, thanks to Hochschule Hannover who are funding a chunk of develoment work that should start next week.