Every herb that has any medicinal quality to it no matter how insignificant is important. In regards to terminalia chebula it may seem that it doesn't play a major role when it comes to hair loss and restoration, but it does its part. It also has to be considered that when one herb is combined with the components of other herbs that they can enhance each others powers.
Terminalia is a tree that yields a small fruit much like a nut. In some countries this is used as a food staple and in the Avurvedic treatments it holds a high place. It is used for many ailments and its properties are recognized as being anti viral as well as being astringent. Various parts of the terminalia is used to treat specific ailments.
When we look at what it has in common with the other herbs specifically used in the treatment of hair loss and restoration it would have to hinge around its ability to fight the E-coli bacteria. This is a bacteria that is renowned for creating bladder infections. So once against it is having a indirect effect on the urinary system. Although this has no direct link to hair loss it would seem that any of the other herbs that were beneficial for the urinary tract were also beneficial for the hair.
What is probably the most important aspect of Terminalia is that it will produce chebulagic acid. As this goes through many processes it produces a form of gallic acid which in turn is most favorable in treating psoriasis. Again a significant factor of skin and hair having many similarities it supports the potential benefits of Terminalia in respect to hair loss and damage.
In addition to this gallic acids is also found it many potent foods such a black berries and white tea for example.
Finally when you took a look at the many ailments that Terminalia is beneficial in treating many of those ailments have different degrees of effect on the hormones. Again another fact that forms a link to hair loss.