Yea no I'm also left scratching my head. I've never heard of a methodology to attribute market cap portions to business units; eager to hear some clarification too.
Typically in mergers, you see accretion due to the removal of duplicate functions on the cost side (you don’t need two HR departments for example), but sometimes you get negative credit if you’re a too big / too opaque / too confusing.
Not an authoritative source. No rationale given.