Dirk Hanss wrote:
"Aikido is a martial art as taught by the founder O'Sensei Morihei Ueshiba and his disciples".
Who does not accept this?
Korindo Aikido followers, who acknowledge some connection between Sensei Minoro Hirai (Korindo founder) and Ueshiba but do not consider Hirai to be a follower or disciple of Ueshiba.
I believe there are some other groups of similar concept. People who study Aikido martial arts that may be connected to Ueshiba in some way or not (heard there is some Daito-ryu derivative also calling itself Aikido).
By making such a definition, you are already making some assumptions not everyone agrees with.
If you wonder about the history of Korindo, please search other threads in this forum and in E-Budo. As it is off-topic.