I do think Mother Art is a catchphrase used to sell form DVDs.
You are right in that don't regard hearsay as fact if there is no evidence to support it.
As for the concept of martial arts that are ancestral to others, there is no doubt that those exist. One thing to keep in mind is no culture ever sprang out of nothing, Follow the Vajra back through history and you will find a very old "mother art" that pre-dates Buddhism all together. All you need to do is look at the facts and evidences, ignore claims, forms, nationalist scholars and people trying to sell you something or arguing out of a conflict of interest because of their pride in their system, nationality or commercial interest.
It is too big a picture for those with the narrow view to be able to see. Everyone wants to claim to be the original, the real, etc. It gets to the point of absurdity where people ignore evidence and fact and regard hearsay as proven fact. People become so arrogant they think that they can know what and when in regard to martial arts just by examining modern claims and forms, worse most of them only look just enough to support their own conclusions and never test them against various evidences.
If you want to trace a very interesting influence on Chinese martial arts, as well as Thai, Malay and others, look into the period of the 1200s, in the 13th century a Temple dedicated to Shiva existed in Quanzhou China when the Indian and the Chinese had extensive trade via ocean. Very specific martial arts teachings are linked to the Shiva tradition and the temples. This has a lot to do why the traditional Wai Khru of Thailand (which relates to Muay Thai) involves the hindu pantheon including Shiva. This period of time also relates to martial arts in numerous parts of southern Asia including Silat.
Or if you want, trace Yoga back through history and it will lead you to the same "Mother art" that the Vajra will. Shiva is called lord of Yoga, Dance and Martial arts for a reason. Both the Brahmans and the Kshatriya had their own martial arts systems that related directly to their cultural obligations and spirituality. Standing Gong is extremely well known in Hindu systems and is used by numerous mythic warriors to obtain special powers. A great many of the Chinese mythological concepts are just translations of earlier Hindu concepts, much as is the case with Buddhism, which is linked to Kshatriya culture.
The evidence and historical record in terms of sites and artifacts is far more compelling than any hearsay i have ever heard.