There exist, as far as I know, only three European shibu of the KSR. In 2008/9 when I was in contact with the shibu in Frankfurt/Germany they indeed required to drop all other arts entirely: No aikido, no kendo, no other koryu budo, even no boxing or tai chi.
Maybe things have changed since then conerning other arts. aikido is still excluded.
Yes, I think this indeed is true nevertheless.
A complicated, all too human story maybe ...
Some years ago when I was in Japan I was given an intro to the KSR group practicing at Tokyo University and went along on a Saturday to see if I could train. By chance Seki sensei (KSR Shihanke) was in town, and obviously he took the class. When I asked if I could train, he immediately asked if I was Tanaka sensei's student and I replied that I was Inaba sensei's student. He then said that he had had some other people come asking to train, and he had sent them away to get permission from Tanaka/Inaba (and they hadn't come back). When I mentioned that I had already discussed this with Inaba sensei who had no problems with people training elsewhere, he thought for a moment and then said that if I took the responsibility, he would allow me to join the class. I enjoyed it, and we had a beer and nibbles afterwards - very friendly group. I was given permission to return the following week "but only for kihondachi". Unfortunately that was the week I had a kidney stone (high summer and not hydrating enough while practicing - watch out!) so I was unable to
Moral of the story perhaps is that life can be easier if you talk to the head guy
On a related note, I have had a very pleasant lunch with Karl Friday on a different trip to Tokyo.
I would be happy to train again (but not if I had to renounce all other arts etc...)