This is a lot of extra complexity they are inflicting on themselves if accurate.
That may be true for the M3, but they undoubtedly are already working on the M4 and M5; they may have bigger differences planned for future releases that require the Pro and Max designs to be separate. Could be that they are just doing the initial work to split the design into two branches for M3 so they can make more changes from M4 onwards.
if this extra complexity is mitigated by hiring a larger, separate team (rather than strain existing engineers), then they might win overall because of the customer segmentation.