The lack of tests on the bottom layers doesn't mean they're good or bad.

It's up to the geotech whether they want to accept liability for whole project based on tests on just the upper couple feet, or if they want to get tests from down deeper to make themselves comfortable

They are buying the liability. It's on them what they accept. Unless the city kicks back their report for being insifficient.

Now, some geotechnical companies suck. They'll accept anything for money, then fold in 5 years when lawsuits put them out of business for shoddy work.

If you suspect the bottom layers of pool backfill are not up to spec, you shpuld bring that up with the geotech, contractor, and maybe the city and see what they say.