When you read a "study" you need to dig a little deeper and read between the lines. Anyone can create a study to support their agenda. This study did not say that IPL produced a dangerous amount of lipid peroxides and did not clarify if any products were used on the skin. You should always look for something called P-value at the end of a clinical study - google it.
Are bodies are subject to free radicals and oxidative stress all day. Eating and breathing introduce free radicals. Oxygen can cause oxidative stress - I guess we should all stop breathing.
Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism and the damage they cause to cells is constantly repaired.
Short-term oxidative stress may also be important in prevention of aging by induction of a process named mitohormesis. Reactive oxygen species can be beneficial, as they are used by the immune system as a way to attack and kill pathogens.