This is a detailed study of the immune response to vaccination for one specific group of immunocompromised people, those taking anti-CD20 monoclonal antibodies. This study was done in people with MS, but these monoclonals are also used for lupus and some other autoimmune conditions. Because CD20 is found on the surface of B-cells (which develop into plasma cells that make antibodies), the drugs that target CD20 dramatically reduce or abolish the antibody response to vaccines.
This study finds that all those who were vaccinated, even those that could not make antibodies, were able to make CD8 T-cells specific for the spike protein. Interestingly, their immune systems appear to be compensating for the lack of an antibody response by making even more CD8 T-cells than healthy controls. Although CD8 T-cells can't stop an infection from happening, they are good at killing virus-infected cells in order to clear the infection once it has happened. So there will probably be a benefit in terms of reduction of the risk of severe covid and/or death for these people, even if their antibody levels are zero.
So that's good news for people in the specific situation of needing to take anti-CD20 antibodies. It's also a cool bit of research showing that the immune system is sometimes smart enough to work around deficits in one branch of the immune system to compensate elsewhere.
I don't think that we can translate this to all immunosuppressed people. There are lots of different ways that someone can be immunosuppressed, and if your immunosuppression is targeting your T-cells, I wouldn't expect to see this same effect.
And also, this doesn't mean that people with past covid who didn't make antibodies necessarily have CD8 T-cells to compensate. A different study previously showed that CD8 T-cells in those with past covid tend to wear off more quickly than their antibodies, probably due to covid's targeting of T-cells. So if you had covid, the evidence so far is pointing toward antibodies as the best measure of your level of protection.
Link to study : https://www.medrxiv.org/content/10.1101/2021.06.23.21259389v1.full.pdf