Yes, Graham, Buddhism came from Nepal-India, but it traveled through the filters of China, to Japan. That's why I used "intro." Furthermore, the foundation and roots of internal body methods now practiced in the Far East may also have been derived from practices in the Near East, but again... through the filter of China they went to Japan. We're talking a couple of thousands of years for ideas to evolve and be adapted into other cultures.
My point, though, is that the methods that drive IP and aiki did not, themselves originate in Japan. They came to Japan from elsewhere. China's huge influence over Japan, and its own various arts and disciplines, points to it as the source. Where China original got its inspirations for various things, is another topic.
The most recent incarnation of what we would
The methods that drive IP yes but Aiki no. The methods that drive Aiki are spiritual and not of chinese internal mental/physical methods.
Spiritual Aiki is driven by spiritual disciplines borrowed by whatever person of whatever nationality if spiritually aware enough.
The mind is driven by that of the swordsman as are the movements physically.
Many Japanese concepts quite alien to chinese exist in Aikido. So influences pertinent are purely Japanese from bushido to budo to kotodama and shinto and more.
Spiritual has no nationality actually for it is universal as are the basic principles of Aikido.