They are so good! 😭🥰 It's just Akiko isn't singing here. (There's a studio version of this song where they're singing duet though.) But perfect nonetheless!
I think it's a fusion of both Taeko's and Akiko's versions. It sorta has the same tempo as the original, but it has the lonely piano-only vibe of Akiko's 1996 cover. Also I believe the song really is a bittersweet one based on a Google translation of the lyrics (sorry I'm only a beginner in Japanese!), talking about a former friend (or lover?) and the good times they've had.
It's intentional. The text is extremely melancholy, but the original recording on Mignonne is painted with Ohnuki-san's very distinct through-arranged city pop sound. When Yano-san recorded her version in 1996, it came off the heels of her untimely separation/divorce from Ryuichi Sakamoto, which happened in 1996. Listening to the recording, you hear how the internalization of the separation has broken her heart as she fights back tears.
