I'm going to assume that the adoption has been done according to the proper legal procedures (i.e. other parent accounted for, either by absence/death or having not objected to the adoption).
-The children are already PRs. They have already relocated. There is, therefore, no concern of adoption of convenience.
-There is a specific exception / reference to the case of a married couple, of whom one parent (the stepparent) adopts the children of the spouse.
So no: you're wrong. If everything is as described by the OP, there is no issue. It's a common procedure that is specifically recognized when a step-parent in a married couple adopts the spouse's children. There should be no concern that it's being done for improper purposes since the children have already become PRs.
WIth one valid caveat, that assuming this adoption procedure in Taiwan done according to law and acceptable to IRCC. I've no reason to believe otherwise, and neither do you.
Adoption is not always done because a parent is deceased or losing parental rights in other countries. The goal seems to be to secure citizenship through the parent right away and not wait 3 years so there is definitely a concern that the adoption is motivated by securing citizenship right away which would fall under adoption of convenience. We need more information about the biological mother.