You are correct. A woman’s foot or leg does not need to be off the ground or out of contact with the mikveh wall (or stairs) when she immerses. This is because the waters of the mikveh already cling to it and reach the leg to at least a small degree, even when it touches another surface.
Please do follow up on this matter with the attendant, so that you and other women need not perform extra dunks in the future. If it is not comfortable for you to address this directly with her, you may wish to reach out (even anonymously) to the mikveh’s supervising rabbi.
Please feel free to get back to us with any further questions.