Well aside from this guy wanting to teach Martial Arts over the Internet or Correspondance courses I have a few problems.
One he doesn't actually document anything, but I certainly can agree that MANY if not MOST Japanese Martial Arts came from Jujitsu. However they added other techniques and modified them and became more specialized and because different.
Just like ALL medicine comes from Hippocrates... the fact that it has been heavily modified and evolved makes today's medicine almost entirely different than that of which Hippocrates used, it is like opening a school for Medicine using entirely the Hippocratic system of medince and humors and stating "Why learn that other medicine, it all came from Hippocrates!"
But more importantly... ALL Martial Arts are not Japanese.
I can promise you Mongolion Folk Wrestling has no influence from Jujitsu.
Muay Thai kickboxing has no ties to Jujitsu.
Capoeira.. no ties
Afrikan fighting arts--no ties..
Pankration has no ties to Jujitsu, etc. etc. In fact if you believe the hype of other people selling their art, one could argue that Jujitsu and most Martial Arts came from Pankration, and the conquering of Alexander the Great. That his greek soldiers taught Indians hand to hand Martial Arts, Indians in turn taught the Chinese, Chinese in turn taught Japanese...etc. etc.
The truth is there is no father of all Martial Arts, and if there was it certainly wasn't Japanese in origin, as their trade routes were small and influence globally was minimal until two hundred years or so.
So honestly these kind of claims are had to make, and pretty much impossible to substantiate, especially given that there are Martial Arts from all over the world, that have no ties to Jujitsu.
Additionally, this "source" doesn't actually give any historical documentation, other than his own words. He is a Godan, and he is trying to sell people Jujitsu online...
I would be wary of anyone trying to teach you any art online, via correspondance course, or video series, and I certainly don't consider their word gospel.
It's not that I dislike Jujitsu, I agree with a lot of what he is saying.. as Jujitsu being the father of many JAPANESE Martial Arts. I would disagree with it being the grandfather of Kung Fu or Chinese arts.. (that have been around far longer than Jujitsu, and are in fact believed to be the source of Martial Arts introduction into Japan and Okinawa). Nor does it have anything remotely to do with most other Martial Arts that are not of Japanese nature, aside from BJJ. BJJ came from Judo, Judo came from Jujitsu, yes we all know that.
Many Karate masters cross trained with Jujitsu before founding their own style of Karate... ok.. I can agree to that.
Aikido came from Jujitsu, yup I would agree with that. I don't think anyone would argue against those claims.
But Kung Fu? Not buying that.
And again... "ALL Other Martial Arts" again would completely disagree with that, shown as above.
But hey, I'm not trying to sell anything. I have just done some hobbist study of the history of many Martial Arts, Hoplogy, and a little bit of common sense.