Thats not a repeat. You originally asked why it’s a problem since you mistakenly thought it’s not assur to bake chometz on erev Pesach.
Now you’re asking a new question – why don’t we say it’s muttar because it’s two d’rabbanans.
The simple answer is that usually even two d’rabbanans is assur. In this case, they obviously accepted the minhag that way, as we find that people abstain from kitniyos at the same time as chometz.