Manoah has already had a season better than any of Stroman's with the Jays, and it was in his first full season in the MLB. Obviously nobody can really complain if he posts consistent 3 WAR seasons like Stro did, but at this point in time his ceiling should be considered higher than that.
I think people are taking the "Manoah will probably regress" and seeing his early 2023 results and thinking he's some mediocre pitcher. Manoah has been exceptional early in his career, not just by ERA. He still has room for growth, unless something terrible has happened, he should go back to 2021/22 levels with a chance to improve on that.