Because the flashback is from Okaya's perspective, this is probably another unreliable narrator moment for Okaya.
There is probably some kernel of truth to it. But we may never know. It's already hard enough to get the perspective of a normal kid, let alone a psychotic one.
Yeah, it's already a stretch to expect Okaya himself to immediately notice or just intrinsically know that Kiri was recording their conversation (since when? presumably they went from the school stairwell to Okaya's house so was Kiri recording the entire time?) let alone the fact that Kiri would have the wherewithal to record to begin with but now you also include that Kiri is apparently an expert psycho analyst who got Okaya pegged immediately and saw through his bullshit when both of them are like fucking 7 years old.
Not only that but Kiri was never portrayed as anything approaching this intelligent in the early chapters, his whole schtick was always just that he wouldn't bow to pressure from Okaya which is why Okaya had gay psycho love for him. The only consistent characterization between early neutral PoVs and this current story is Okaya being fascinated by Kiri and Kiri being stubborn/a foil to Okaya. Everything else should be taken with a huge grain of skepticism.
Barring that, if it turns out this IS what really happened then I would (possibly hot take) consider this
worse than Juujika because at least Juujika was dumb from the get go while this had potential only to go for the low hanging fruit of "you made me this way" villain origin story.