Sounds like probably just a meme assumption, Aum Shinrikyo technically isn't even the same thing anymore, having split into the PMC sounding "Aleph" and "Hikari no Wa", the latest big thing either did was an Aleph guy driving a car into some people in direct retaliation for Aum Shinrikyo members being executed back in 2018-19.

Theres most likely other right wing extremist groups and cults active in Japan that arent as famous, alongside the potential that this guy is just a lone nerd weirdo who built his own gun to kill someone famous.