While it is true the Mishnah Berurah says that woman in current times are all considered important and hence all are supposed to lean basic Halacha does not require women to lean so we do we say KULANU MISUBIN (WE ALL LEAN)?
All does not mean ALL. There are other exceptions apart from women like a student by his teacher. It means all who are supposed to lean. They lean and dont sit.
This crux of this question is asked by the Natai Gavriel. In a nutshell, his answer is leaning is about showing the autonomy and freedom of liberation, and that we all participate in the action of showing the liberation of leaning, whether or not an individual person leans in that situation. The non-leaner's actions are part of the demonstration of freedom of the group which contains someone who's presence prevents the leaning.
So we are all participating in the demonstration of freedom that leaning represents.
This answer is a modified version of this one to fit the format of this very similar question.