I don't think this is accurate. MGM inherited rights to Mad Max from its acquisition of Orion Pictures, the successor company to American International Pictures, the original North American distributor. However, MGM now has a physical media distribution relationship with WB, which replaced the previous agreement with TCFHE, and so negotiating for a box set such as this is simplified.From what I’ve been reading in this forum I have reason to believe that the North American rights to the original film have reverted from MGM to WB, which means WB now fully owns all four films in the series, and which also explains why the supplements from the earlier releases are absent from this new collection.
If you have bought the previous Kino 4K release, you may already have a collector’s item in your hands. If not, seek this out before it is too late.