From what I understand this uses overdose numbers. They should also not be assumed to be comparable across countries (recording practices vary widely), but if they tell the same story as wastewater monitoring there may be something to it.

Was that because there wasn't much fentanyl use or because fentanyl metabolites are somehow harder to detect?

If it's below the noise floor, then you didn't really detect it, did you.

Does wastewater drug monitoring account / offset for constipation caused by drug use?

There isn't anything to measure if the poop is unable to leave the end-user.