Attempting to hijack your brain's reward centers as a way to succeed with a meditation app is kind of tone-deaf. At the same time I don't think it would be successful either - does anyone really care about building a fictional garden? The whole things strikes me as doing the wrong thing and probably failing at it too.
They should have leaned more into the journaling / gratitude /overall mental health aspect - at least that's kind of a differentiating feature.