Yes, I know that, but my question is really about how canceling a score might help someone meet specific entry requirements, since it just takes away a score and doesn’t add a better score.
My understanding is that the requirement is to have some number of 5s on APs (or either 5s or 4s depending on the uni), but I don’t know of any specific requirement to NOT also have a lower score on some AP exam?
It doesn’t. They require all scores, including cancelled ones. Some applicants think they can be clever by cancelling thinking the uni won’t notice or won’t discover the commission. The reality is they’re only providing the opportunity to get bitten in the butt
I guess I must not be explaining myself very well 
For example Imperial’s requirements listed here (Accepted qualifications | Study | Imperial College London):
Minimum of 3-4 AP tests with Grades of 5 (see course pages for required subjects). Expected to be taken alongside a relevant high school diploma.
Where sat independently, departments will seek additional AP tests 4-6. Where taken in conjunction with other qualifications, departments will seek the required standards in all qualifications presented.
So the minimum requirement is 3-4 (or 4-6) AP tests with a score of 5.
AP tests that do not have a score of 5 would be simply irrelevant to meeting this specific requirement, regardless of whether the score was canceled or was a lower score than 5. So I don’t understand why the OP thinks canceling a score would help to meet a requirement.
It may be a little like Georgetown requiring all SAT scores. There is a difference in showing a 5 for a particular required AP the one time you take it (as most who report scores do) vs taking it a second or third time to meet the requirements. (Maybe not clear from your extract but it’s not 5s in any subjects but specific ones for specific courses)
I don’t see any downside to waiting to see the score- and a couple of upsides.
Does your kid’s HS have massive grade inflation? Well, she got an A in AP History and a 2 on the AP exam. That’s going to help her figure out where she stands on a nationally normed basis of college going kids- not just in her own HS where she’s likely the Val and the top of the academic heap. Your son’s counselor is trying to help him decide if he’s ready for the more intensive “physics with calc” or the less intensive. He self-studies for the Calc AP, gets a 2, so it seems clear that the physics with calc is NOT his best option for next year.
Once a kid pays for and takes the test- unless they walked in to the center with a massive migraine or a throbbing toothache and couldn’t concentrate, I don’t see a downside in keeping the score. And it may help the kid make better decisions for the upcoming year.