Yes radical feminism is based on the fact that women are discriminated against and worse because of their sex, ie women as a sex class are oppressed by the male sex class.
But that does not mean its simplistic, it means that irrespective of your race, religion, class and so on, women still share a common experience of oppression by men. But what radical feminism also recognises, as it is about grass roots activism, ie life as lived not theoretical concepts, it is about autonomous action and thought.
So this means radical feminism with its common understanding of sex as the basis of women's oppression allows / encourages women with other commonalities such as race or class to work together to provide a framework for how to organise based on the overall common oppression of sex and another common shared oppression such as race.
In theory this allows women to work together as suits them rather than some top down notion of this is what you should do or think, ie at any time or most of the time women can work autonomously as a single issue group, but can also them join the larger group on a shared concern eg abortion.
The practice of interesectionality is presented as these positions being competitive and lead to as much time being spent on competing as to who should acknowledge this or that group as though it were some a constant jostle for power positions.
Intersectionality as banded about is very much about academic theory and gives some women the opportunity to endlessly write and pontificate, and talk down to other women for not sharing their concepts.
Whereas radical feminism (in theory!) means that at any one time disperate groups of women can come together on a shared issue, as well as continue to have the right to organise around the issues that are their priority.
Obviously that doesn't guarantee that in shared spaces or campaigns racism or classism isn't an issue, but in aiming for non hierarchical activism, it shouldn't become so easily entrenched.
However I think neither concept of feminism has really survived the surge in queer trans politics.
Intersectional feminism embraces it and then become submerged, whilst radical feminism doesn't, not because of transphobia but because what it proposes is totally the opposite to the basis of radical feminism. Women as a sex class.