Doing a bit of research, this applies to what’s considered a “manual search” as in a CBP agent spending time manually looking at your phone. It does not appear to allow (or speak to?) plugging your phone into forensic tools for a deeper inspection.

Not great, but I think that’s what’s being specifically adjudicated here.

They still cannot deny an US citizen entry if you decline to unlock your phone, though they may confiscate your device. I don’t know if they would then be allowed to do deeper forensics without a warrant.