Yes, you are reading it properly in that a tennis ball could help alleviate the pain from sciatica.
A tennis ball basically apply’s the rules of massage, acupressure and reflexology which can help to relieve sore muscles and muscle tension. For sciatica pain, the tennis ball works on the piriformis muscle which is located close to the sciatic nerve. This muscle can push the sciatic nerve against the tendons beneath it which triggers the buttock pain and the pain down your leg. It presses and treats trigger points in the piriformis muscle which reduces the muscle tension and improves mobility and blood circulation to the area.
They say that it is also good for back pain as it acts as a massage to help reduce muscle tension and provide relief from lower back pain. Of course sciatica is a serious spinal problem so before you embark on any treatment for it you should check with your GP that this type of treatment would be of benefit to you.
I have regular trigger point injections into my piriformis muscle which is done under xray in theater. I find this really helps my sciatica but doesn’t last long enough and as I can only have them three times a year, I have always been on the look out for something else to help with this type of pain.