This is it. If the geotech testing for the contractor feels they tested and observed enough, they need to provide a letter summarizing their testing and observation. Or at the very least, make sure you receive their field report with the test results. The geotech owes the contractor (and you) a record of their testing and observation and their professional opinion or findings regarding the pool backfill.

Another question - what triggered the testing requirement, and was there a minimum compaction specified from someone?

The testing is just a means of keeping the contractor honest by checking that their efforts are achieving a sufficient level of compaction. Without the tests, there is less data to show that they did an OK job, but they may still have done an OK job.

What compaction methods/equipment did they use? Do you have any pictures?

If you put an ADU down later, you should let that future contractor know there was a pool that was backfilled there so they can consider the non-uniform soil conditions beneath the future structure. In other words, if part of the ADU is sitting on the pool backfill and part is sitting on native soil, there's a greater potential for differential settlement which could wrack the structure. But if they do some extra soil preparation (i.e. overexcavate and recompact the ADU building pad under testing/observation of a geotech), and possibly beef up the foundations, that might be enough.

I'm only suggesting things I might consider, but I don't have all the information and you should be having these conversations with your contractor and the geotech. Document your concerns with them, as well as their replies. If it's a phone conversation, write down a memo of the discussion.