CBC's coverage says his problem isn't with the law itself:

Blanchet's criticism follows Carney's remarks Monday that a government led by him would act as an intervenor at the Supreme Court of Canada should it ever hear a challenge to Bill 96.

Carney said he would do so not because he has a problem with the legislation, but because he opposes any province's pre-emptive use of the notwithstanding clause to pass laws.