The range of responses to HIV has always been very wide. Some people become ill within 1 to 2 years of infection. Others can have a strong immune response that lasts for many years.
If your CD4 count stayed above 500 for more than ten years without ART you a long-term slow progressor (LTSP). If viral load is also undetectable, this is called an elite controller (EC).
The benefits of ART are now thought likely to be important for people with LTSP and EC responses.
This is because HIV may have been affecting other parts of your body than just your CD4 count.