Yes, sorry 'bout that! All I can say is moodle's question bank is very .... can't stress that word enough ... complicated.
But, there's another way to approach this issue ... might be better too!
You have a .mbz file ... the backup of the course. Copy the .mbz file to a test directory. Un-archive that .mbz in the test directory. Then open questions.xml file and look for the ID number that matches or that phrase you saw.
Remove that question only .... an xml file has beginning tags with data ... the data ... then an ending tag. Make sure get whatever encloses the question in questions.xml
Once you've done that, we need to re-package the backup ... from inside that test directory, remove the .mbz file and then issue:
tar -czvf backup.mbz ./*
That will re-create the .mbz file with structure that moodle understands is a valid .mbz file it can restore.
Good luck!
'SoS', Ken