That part of CiLR is actually the opposite; the udonge flowers in the presence of impurity but it still hasn't bloomed despite being years after IN, suggesting there's something else at play. In the past I think people wanted to attribute this to some kind of tangible effect, like Kaguya being a well of purity (which isn't a real concept), but I don't think that's the case at all anymore and curse my past self for poor reading comprehension.
Rather, the chapter overall is about the comparison of Kaguya to Eirin and Reisen, who have both moved on from the Moon and are now doing as the Earth people do. Kaguya on the other hand has not found a purpose for herself or motivation to participate in Earth society, and shelves her worries off to Eirin. The unblooming udonge bonsai is a metaphor for Kaguya, as just in the same way, she hasn't yet bloomed years after being in contact with the Earth. There are many passages in the chapter that allude to this both from the side of Kaguya and the side of the udonge, some of them more obvious than others. Kaguya even wonders why Eirin named Reisen "Udonge", and it's exactly because of what she speculates -- that Reisen would flourish once being touched by the Earth's impurity -- but she can't make that connection with herself. At the end of the chapter, Eirin challenges Kaguya to find something that she wants to do, and Kaguya ponders that perhaps something would happen once the udonge blooms, or rather, that it would bloom once she found that something for herself. Once you go into the chapter looking from this lens it becomes very clear.
