Shuichi’s assertion that a 10,000 members strong organisation cannot exist is [based on his limited perspective] to begin with, which Ouma has already addressed, only to have Shuichi repeat the same nonsense. That would be like going to Byakuya and asking ‘say what’s this secret society called? I’ve never heard of it, are you suuure this isn’t just a joke? I can’t think of anything that fits’ when no shit, any information connected to a secret organisation is locked behind archives and is dangerous enough to know that you could die for it.

Moreover, he’s not giving any basis besides that, for why Ouma’s Ultimate Supreme Leader title ‘doesn’t mean anything’. In fact, this entire response started with the express purpose of ‘disproving the nutty claims made by this nut’. He’s chasing the proverbial ‘stain on Ouma’s back’ - that is the ‘lie’ - which is why he’s questioning the existence of the organisation, because to Shuichi, if he doesn’t know about it, then it must not actually exist!

It’s the [same nonsense that Ouma had to dress down Kaede for], but it’s framed as if Shuichi is the one exasperated with silliness and being ever so patient putting up with it. Of course, Ouma’s demeanour is what does it, at this point in time; he looks like a child, behaves rather like one, and self-professes as a liar. He also doesn’t admonish Shuichi in the same way as he does for Kaede, so it appears like that he’s yanking his chain and that’s that. But demeanour alone is NOT grounds for proof, and judging from that alone is poor form for a detective.