Updated Wikipedia page just yesterday:
MUJI was transferred to Ryohin-Keikaku in 1990, and is not part of Seiyu.
Walmart bought Seiyu a few years back, which created the assumption last week, that Walmart now owns MUJI. But, according to the updated Wikipedia page this is not correct.
Now your know and can continue your hipster fantasies. Phew… pass me a PBR.