Although i haven't seen any medical evidence pointing to hair loss due to reason of excessive masturbation, i feel using my common sense, that the statement does hold true in some ways. When a guy masturbates, the level of testosterone increases dramatically in his body. As scientific evidence have proven that hair loss's main culprit is the presence of large amount of testosterone in a person'a body. So you can see there is a link that by increasing the level of testosterone through masturbation, the rate of hair loss in a person also increases.
However, this only applies to person who are experiencing hair loss and people who are not having hair loss are not affected with the increased level of testosterone. People without hair loss problem can have a very high level of testosterone in their body and still not experience any hair loss. The reason being that their hair follicles are programmed genetically not to be affected by the presence of high level of testosterone. Testosterone also cuts off the amount of supply of oxygen and essential nutrients going to the hair follicles. As a result the growth cycle of the hair follicles is affected and the new hair produced in every hair growing cycle will be finer and finer until the time when you can see the hair no more on that particular bald spot on your scalp.
No wonder you can feel the amount of oil and dirt your bald spot on your scalp collects every time after masturbation. Those are the after affect of excessive masturbation. By reducing masturbation, the amount of testosterone is reduced which in turn curbs hair loss.